If they look and "feel" fine/healthy I wouldn't worry to much about it.
Sound as you already have them on a great diet.
Again, if they look and feel healthy, they most likely are
Maybe you could pick out the oat grains from the mix for a week and see if it makes a difference, if you suspect them to be the reason
Also, what time of the day did you weigh them?
If after their "sprouty" breakfast that would account for a bit of extra weight I would think as its a bit more "dense" than just dry seeds?
Just my 2 cent.
Edit: and since you just started weighing them again, you might have caught them at a "bad" time, maybe results will be different next week
Edit2: also it takes a while for the new diet of "healthy" to kick in I would assume, cannot imagine this diet making them gain weight!!!