Edit: I see that Mangadot counts previous ".5" chapters as a +1. Probably the mismatch comes from there.

True! I'm not sure where's the mismatch. 304 is the chapter number given by Mangadot (while The Koi Pond has 234). I see that earlier chapters on Mangadot have, for example, "297 Mag version 227", which seems to refer to this +70 discrepancy... Earlier still there's even "275 Official Scans 213". Go figure!