Spells/effects are calculated on their release, not on hit. And since the spell reflection buff is up during the release of more then one spell it will also reflect these spells.
Crits, dodges, ect is calculated that way but not a reflect. A reflect is calculated when the spell hits the person who is reflecting it, it's made so on purpose to you can reflect a spell that is already in flight if you are fast enough. Some spells got there flight time speed increase (moves faster through the air) due to this some 2 or 3 years ago.
It is now not possible or extremely rare (like 0.01% rare) that 2 spells that are casted will cause this. Although it happens alot with casting+instant in that order due to que times of spells.
i use this to my advatage when i get hit by freeze or frozt nova, to make sure a mage is not stupid enough to try and fake cast and only pop spell reflect the mili second the frost bolt cast is finished. That followd by a hc leap will usualy scare a bad mage into a blink charge + Hamstring = death.
same works in reverse, if the spell reflect wares of while a spell is in flight does not get reflect. i have never been able to confirm a reflect against an already incoming spell when spell reflect wheres off just after the spell has been released.
let me say it in another way that might make more sense. It's all about qeue times.. Spells that get qeued together to overlap will cause both to get reflected.
I suggest you get an addon like 'Tell Me When" and set it up to show and icon on your UI when your target has reflect up so you can either not cast or atleast not qeue spells.
Mages has lots of cc and reflect on warrior is only 5sec, frost nova lasts for 4 so it's not a really big deal to wait.