I also saw a quote where it questioned giving "wealth" as a monthly figured. That wouldn't make sense, I agree. But when you're applying for something where you'll be making monthly payments then asking for monthly income is normal.
Tithing, I can answer how it was 40-50 years ago, but don't know if this has changed in any way. They had an annual meeting sometime around this time of the year where you'd make an appointment with the bishop and would come in with appropriate paper work for the last calendar year (W2 forms, tax forms, whatever) and show that your income was $X, that your accumulated payments for tithing that year were $Y (maybe you'd pay every payday or every month or whatever worked for you). If the accumulated payments weren't 10% of the total income you'd presumably write a check for the difference to get you to 10%. This process is known as tithing settlement. But it really has absolutely nothing to do with why some places ask for or some people give their salary as a monthly amount rather than annual.