You grossly miscalculated because you forgot that money has to actually be in the account to earn. You earn nothing on the £50 you haven't deposited yet. And that is the first problem: compounding isn't working for you because compounding takes time, and only 600 quid is even in there for 4 years.
You're getting fleeced
Adjust for the above, and the return is more like 20% or 4% per year...but that itself is pretty lousy. This is a terrible investment.
I tried a few different numbers into this thing. Increasing number of years yields a disproportionate return, and that tells me they are chomping off a fairly substantial "front end load" that seems to resemble the 5.25% that is typical in the traditional "sucker bet" investment industry. So every month only £47 is making it into the fund, for a total of £2843 actually invested. Obviously that consumes 5% of your returns, but it's worse, because that 5% also doesn't compound over the 5 years, eating another 1% in your scenario.
Given the dismal returns, it is also obvious that they are placing your money in traditional managed mutual funds with about a 1.5% expense ratio. That means every year what you would have made, you make about 1.4% less. That guaranteed total loss compounds too, so adds up to about 8% in 5 years, except you only invested for half the period, so more like 4% over the 5 years.
So, the 20% you did get get plus 6% load lost, plus 4% expense ratio lost, puts us at 30% which is still sub-par for 5 years. Where else is it going? Well, Investify is providing you a soup to nuts, turnkey solution, that "pretties investing all up" for you, makes it seem acessible, and takes the fear out of it. Investing for morons: throw money at them, and they invest it. They are gouging you for a couple percentage points a year for that "service".
... (on top of the ~5% kickback since the front end load is mainly sales commission for the broker, plus some of that 1.5% expense ratio if they manage the mutual fund, and I have seen broker-managed funds that brazenly charge 1.5% expense ratio for an index fund).
Also, there is a fair bit of overhead per transaction, so investing so frequently and at such a low value as £50/month means transaction costs will be a worrisome fraction of the investment.
All in all, the various fees, loads and expense ratios are sapping you for half the gains your money actually did earn. This is stacking with your initial error, to make the gains look awful.
This is the financial services industry working as intended. It is designed to convince people not to think or learn about investing, convince them to blindly trust the industry, and then let them have a small amount of profit to think they're doing OK, while actually pocketing most of the real gains. There is a near infinite number of stupidly complicated products designed to snow investors and create more pretenses to sap a little more of your gains.
The mere fact that you're not going "oh cool, 10%" and going "hey, waidaminnit, 10%?" says these hokey products are not for you.
By contrast: open a Vanguard account: free. Transfer £50/mo. into eTrade account: free.
Buy VTI, a US ETF (simply because that's one I can attest, UK may have comparable):
US $7 FREE per trade Because it's an in-house ETF. VTI earns in the robust US stock market, on average about 7% a year or 40% in 5 years.
Now, VTI is an exchange-traded mutual fund. Its front-end load is 0. Really. Its expense ratio is not 4%, not 0.4%, it is 0.04% a year. That is £1.20 a year on £3000. Really. So all the fees, loads and expenses over 5 years, will be about £6. All the remaining gains go to you.
VTI contains every publicly traded US stock. The fund makes no attempt to pick stocks, it simply buys them all. As such, its expense ratio is very low; as compared to the typical 1.5% of professionally managed funds that try to beat the average (and aren't likely to beat it by more than their expense ratio, so, a net lose).
The concept is described in John Bogle's book "Common Sense on Mutual Funds". Bogle founded Vanguard, creator of VTI. Several other companies like Fidelity and Charles Schwab do the same thing.
Try it like this. You'll do much better. I can't say I recommend US stocks right now but save your shillings - I think Wall Street may be about to have another half-off sale!