It could just be a runt. Or it could be a mix. Hopefully you will know more as it matures - see whether it has the upright serama stance. I have a hard time distinguishing between poor, pet quality seramas and oegb.
I picked up some pullets that were being rehomed - they were supposed to be seramas. With the exception of one size A girl, my own seramas are on the large side, with most being 14-20 (well fed) ounces. But all four of the pullets I brought home make my seramas look fairly dainty in comparison. So I think these girls must be mixed with something else...not sure whether oegb or Japanese??? My little serama roo seems to like them just fine, and I don't sell birds, so it's irrelevant to me.
Anyhow...you may be able to tell more as she matures a bit more - she's a tiny cutie though!