Perhaps it's this custodian's way of indicating that they are not the ones who determined the amount of taxable earnings. However, in the case were the custodian would normally be able to determine the taxable amount but for some reason are unable to do so for a particular situation, they are to leave box 2a blank and mark box 2b Taxable amount not determined. The only case where it's proper to include an amount in box 2a and mark box 2b Taxable amount not determined is when reporting a regular distribution from a traditional IRA (in which case box 2a must equal box 1).
