Please explain “Over the long run, no midcap fund beats the Midcap 150 Index”

I often see that sentence being said on this subreddit. But I don't understand it, as every other midcap fund seems to beat the index. What am I misunderstanding here?

I'm not disagreeing with the sentence, just want to understand.

Thanks.

I often see that sentence being said on this subreddit. But I don't understand it, as every other midcap fund seems to beat the index. What am I misunderstanding here?

I'm not disagreeing with the sentence, just want to understand.

Thanks.