I just wrestled with this as well, very confusing. But if you look at how Schedule X worksheet that uses these numbers works, it turns out that the first number is entered on line C, the second on line D and line D is subtracted from line C and is zero if D is larger than C. In other words, the total previous distributions is only relevant if the previous contributions are greater than the previous distributions. (In my case, with no previous contributions, there was no point in trying to figure previous distributions.)
