I've recently noticed that the pre-market price is sometimes quite far off from the open price. How is this? I feel as if I have seen a pre-market price change almost 5% in regards to the reported open price. But I cannot find any good evidence. I generally watch the Google ticker which I know is not up to date, but it does show correct data.
If I understand you correctly, you are noticing that a stock's price can change drastically when the time changes from pre-market trading hours to open market hours. This could occur because a much smaller pool of investors make trades during pre-market and after-market hours. When the regular market opens there is a large influx of trades, causing the prices to jump.
If you're talking about an ETF trading on Arca, it's probably because of the opening auction:
The match price is the price that maximizes the volume that can be executed within the Auction Collars. The Core Open Auction will use the match price closest to the closing price of the previous trading day (based on normal market hours) if more than one indicative match price is valid.
The core opening auction doesn't really take the opening session activity into account, as you can see - the market runs an auction and whatever price clears the most volume, within certain limits, is the opening print.