Plan giving Prior to 2016 and After 2016 Data - why?

I have a client who is taking Stock out of a previous employer's 401(k) plan. The data given to us from the distributing plan was broken down between Prior to 2016 and After 2016 amounts and cost basis data. The sources of money are after tax and company matching. Why would the 2016 before and after be significant?  Any ideas? 

as a wild guess, since you said 'shares' it sounds like an ESOP of some type.  it is possible some shares are 'tainted' and other shares are not.

by 'tainted', I mean one of the owners put his shares into the plan and received a tax deduction, and therefore he is not able to receive any of those share if there are forfeitures or distributions which get reallocated.