The number of aircraft you pick up each day can differ due to schedules, the weather and probably an number of other factors I have not mentioned and at the end of the day there is not much between your highest and lowest number of messages picked up
Hopefully as the weather gets better you will pick up more aircraft, for example the better the weather the more GA aircraft are in the air. Different days see differing schedules for example weekdays usually see more scheduled traffic than weekends,
Also as when are able to travel more abroad in the not to distant future we will see even more flights
I dont know where you are located but the weather can make a difference to traffic levels as for example transatlantic flights can move north or south depending on the prevailing weather to take advantage of more favourable winds
The summer months see lots more traffic due to there being more flights going to and from the favourite holiday destinations.
Air traffic control also routes traffic depending on how busy it is at any particular time/day and the routing given can change for any given flight depending on traffic levels
The number of messages you pick up can therefore vary each day depending on these and other factors
Alan
