I dont think its strange at all considering their geological location to have only 3 seasons, Achet (time of flood), Peret (time of sowing) and Schemu (draught), it makes sense considering that the nile was the one thing that kept them alive.
i think if you compare it with our understanding of seasons they could do with only two summer and winter since the difference in the average temperature is like 15°C, also they probably knew of the flood and drought before they invented the calender to predict it, so that could be the reason for that