This confusion has been pointed out by others before.
Have a look at this thread:
https://community.monzo.com/t/android-ios-spent-today-showing-0/52740?u=awjdean
Basically, Monzo take some transfers into the account off of Spent Today, but not others.
Personally, I think only the following should be taken off Spent Today:
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Bill Splits.
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Monzo-to-Monzo requests.
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Monzo.me payments.
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Transaction reversals.