"Why would ProSeries take the exclusion anyway?"

Because there are two different exclusions. One is based on source and has no limit, and one is based on age and source and has a llimit.

"If your pension is taxable to New York and you are over the age of 59 ½ or turn 59 ½ during the tax year, you may qualify for a pension and annuity exclusion of up to $20,000."