Nicely explained sygyzy
Put simply a credited hour is the same as a flying hour. Quite why BA have 1045 and not 900 defeats me right now but no matter. 12 months at 87 hours a month.
AFAIK I think it is because as you and BA Baracas rightly say leave, Duty Free weeks, Ground Training etc also attract monthly credit. If you then had an annual 900
credited hours limit in place then BA couldn't get you working up to anything like 900 flying hours, which is obviously what they are most interested in.
Or, put it another way: 900 flying hours + leave+ DFW + GTdays = approx 1045 credited hours, plus or minus.
That's probably BS but I hope at least the explanation makes sense.