Couple things id try...move the probe to a hotter spot... I was having issues where i had 2 tubs that were pretty much the same and the 3rd was off like 6 degrees kinda like yours... I tweaked the probe placement a little bit and now theyre pretty much all the same...

Id try moving the thermometer probe in the coolest one(you have 2 tubs right).. You might find you just dont have the probe above the warmest part and your just getting a misreading...

If none of that works...when you have your snakes out put your hand on the bottom above the pad in each tub... You probably wouldnt be able to tell the difference in 6 degrees but you can get a good idea how warm they are... If the cooler one still feels pretty warm then you might be ok... worse comes to worse bump the top one up to 93 and the bottom one should fall around 88... Not "perfect" but itll work.