Is this wrong?

I’m on mat leave and just going into unpaid months.
My assessment period is 16th-15th. I got paid £138 this month and I just got my statement to say I’d earned £860 in the assessment period paid 31/08. Which I didn’t? I’ve as a result only received a very low amount compared to if it was correct. My last pay on 14th August was 860 which was the amount they’re saying I got… I’ve flagged a dispute with them and they’ve said they use the assessment period of 16th August to 15th September.. which is when I earned £138… am I wrong? Right? What’s happening hahaha