Summation register behaviour for Plus42/Free42 question

[nickapos]

Hey guys good morning.
I have a question that you might be able to help me with.
Let’s say I want to add the planes my adsb receiver has received for the last week and procure the average per day.
When I add the first number and press Σ+ in my Plus42 I am getting a message “too few arguments”
In order to register that first entry I need to double that first number in register X and Y and then Σ+ is happy.
After that first number all other numbers are entered without this behaviour. The results seem correct to me despite this behaviour so it is very likely i am missing something here.

Is this a bug or is it normal behaviour?
What am I missing here?
Thank you very much