Bit hard to say until “top X” is defined - “top” implies “best” I guess. But even when it is defined, best is what every fund or active manager is claiming to pick, and we see that many of them are wrong.
So the assumption might be that compared to all of an index/market/group of securities, a selection might offer increased volatility, but we probably don’t know if it will offer better or worse performance If the cost of the “selected” fund is larger than the comparable “all” fund, we might assume that its performance net of fees will be worse.
That is passive investing orthodoxy: buy the most diverse and lowest cost investment you can, ie the “all” fund. The alternate approach is to simply pick the securities that’ll perform better than the market as a whole, easy! Though it turns out that’s easier to say than do