370 Should be Cinderace’s max Speed at level 100. :P
337 is the amount of Speed a Cinderace with 252 Spe EVs, a speed IV of 31, and a neutral nature would have. :P
Either your Cinderace isn’t Jolly or you’ve used a Mint to change its Nature, which may still show up as Jolly, but not be so by stat calculation. :P
There is also the possibility that Cinderace didn’t have Jolly and you thought you gave it a Jolly mint, but didn’t, but that is similar to the above
Either way, the solution is to use a Jolly Mint on Cinderace.
If this doesn’t work, and Cinderace really has a Jolly nature being reflected by the stat calc, then by some way less likely odds, you have 132-135 Speed EVs. :P
Source: Mints, I used Pokemon Showdown!’s Teambuilder to do Math :P
Hope I Helped