That’s because advertisers have a specific daily budget to spend. They bid based on your average traffic, and when there is a surge in traffic the unexpected visits are sold for less so the RPM goes down.
On contrary, when you get lower traffic then your visits are valued more and so a higher RPM. Keep in mind though, that I’m not talking about big brands but small-to-middle sized websites.
Over time, as the traffic grows and becomes stable, the average RPM also improves. So keep on working without over-analyzing daily earnings