Russell 2000 Index confusion between performance of index vs. fund based on it?

On my trading platform (brokerage account) I have that performance screen where you can see how your portfolio stacks up against the major indexes. I noticed the Russell Index killing it lately, so back in Q3 2025 I put some of my 401k funds into a Russell fund. To say it has sucked is an understatement. How TF is the index killing it vs. S&P and Dow, but this fund based on it (BSMKS) so terrible? It's the only Russell option my 401k offers.

I am an amateur investor, so be gentle, and ELI5. Why is there such an enormous performance gap from what I see in my brokerage platform and the performance of this index fund based on it? What am I missing here?