@HKehdy - you’re not the first person to wonder about this. We’re actually in the process of updating Self Serve to give a little example of why this happens.
In essence, the Tab balance is updated based on the date you purchased your phone, which will be different from the date your bill gets issued.
Since you purchased your phone on the 25th of the month, that's the date your balance gets reduced each month, but you won't actually see this charge until your bill is issued,
e.g. Since your bill date is the 6th. This means your final Tab payment was billed to you on your Feb 6th invoice.
Moving forward you won’t see the tab charge on your invoice any more.