For some reason, if you receive money from another Monzo account and categorise it, then it is calculated as part of your Summary (ie added to the Summary amount). If you don’t categorise the payment from another Monzo account, however, it does not count towards your Summary.
However, with payments from any other source, this is not the case. Whether you categorise the payment or not, it does not count towards your Summary.
This inconsistency is odd and quite annoying!