What you are describing is a very specific case of the more general principle of how dividend payments work.
Broadly speaking, if you own common shares in a corporation, you are a part owner of that corporation; you have the right to a % of all of that corporation's assets. The value in having that right is ultimately because the corporation will pay you dividends while it operates, and perhaps a final dividend when it liquidates at the end of its life. This is why your shares have value - because they give you ownership of the business itself.
Now, assume you own 1k shares in a company with 100M shares, worth a total of $5B. You own 0.001% of the company, and each of your shares is worth $50; the total value of all your shares is $50k. Assume further that the value of the company includes $1B in cash. If the company pays out a dividend of $1B, it will now be only worth $4B. Your shares have just gone down in value by 20%! But, you have a right to 0.001% of the dividend, which equals a $10k cash payment to you. Your personal holdings are now $40k worth of shares, plus $10k in cash. Except for taxes, financial theory states that whether a corporation pays a dividend or not should not impact the value to the individual shareholder.
The difference between a regular corporation and a mutual fund, is that the mutual fund is actually a pool of various investments, and it reports a breakdown of that pool to you in a different way. If you own shares directly in a corporation, the dividends you receive are called 'dividends', even if you bought them 1 minute before the ex-dividend date. But a payment from a mutual fund can be divided between, for example, a flow through of dividends, interest, or a return of capital. If you 'looked inside' your mutual fund you when you bought it, you would see that 40% of its value comes from stock A, 20% comes from stock B, etc etc., including maybe 1% of the value coming from a pile of cash the fund owns at the time you bought your units.
In theory the mutual fund could set aside the cash it holds for current owners only, but then it would need to track everyone's cash-ownership on an individual basis, and there would be thousands of different 'unit classes' based on timing. For simplicity, the mutual fund just says "yes, when you bought $50k in units, we were 1/3 of the year towards paying out a $10k dividend. So of that $10k dividend, $3,333k of it is assumed to have been cash at the time you bought your shares. Instead of being an actual 'dividend', it is simply a return of capital."
By doing this, the mutual fund is able to pay you your owed dividend [otherwise you would still have the same number of units but no cash, meaning you would lose overall value], without forcing you to be taxed on that payment. If the mutual fund didn't do this separate reporting, you would have paid $50k to buy $46,667k of shares and $3,333k of cash, and then you would have paid tax on that cash when it was returned to you. Note that this does not "falsely exaggerate the investment return", because a return of capital is not earnings; that's why it is reported separately.
Note that a 'close-ended fund' is not a mutual fund, it is actually a single corporation. You own units in a mutual fund, giving you the rights to a proportion of all the fund's various investments. You own shares in a close-ended fund, just as you would own shares in any other corporation. The mutual fund passes along the interest, dividends, etc. from its investments on to you; the close-ended fund may pay dividends directly to its shareholders, based on its own internal dividend policy.