If those two numbers you mentioned - $4400 and $3585 - represent the amount she gets or pays as of April 15th, and not her total tax, then I would look closely at how much was withheld or paid with estimated payments each year. Depending on where the money came from, the withholdings could be very different. For example, dividends and stock sales usually don't have taxes withheld automatically, while wages typically do. Perhaps in the past your father was making estimated payments and your mother didn't know she needed to do them this year.
