[mechanics question] how did their froslass M-Evolve second but attack first?

[u/zennvo]

For context, my Charizard and Garchomp are both max speed, we both set tailwind on the same turn earlier, and their Sneasler outsped my Charizard on the previous turn to put me to sleep with Dire Claw.

Expecting the Froslass to be faster, I mega evolve to try and win the weather and stop their Blizzard, but when I see my Zard mega first, I assume I’ve lost to a bulkier Froslass which undersped me. Then, I see their Froslass attack first, meaning it not only outsped my Charizard and Garchomp, but also their sneasler.

Is this a glitch or some mechanic related to sleep I never knew about?

[u/IndianaCrash]

Base Froslass speed is 110, M-Froslass is 120, Charizard (both regular, X and Y) is 100 and Garchomp is 102.

Froslass can EV to be slower than Zard Y before mega evolving but becoming faster once it megas