Form 4 math not adding up?

[Grant]

SEC Form 4s have two main fields which let us track insider ownership and trades: "Securities Acquired (A) or Disposed Of (D)" (I'll call this "delta"), and "Amount of Securities Beneficially Owned Following Reported Transaction(s)" (I'll call this "amount").

 

Suppose we have a sequence of Form 4s from a particular insider (all direct ownership of the same class of common stock, no funny stuff). Should we expect the amount - delta = amount of the previous Form 4? I've sometimes seen this not be the case. I'm generally only looking at Form 4s in detail when examining potential scams; is this evidence of undisclosed insider trading? Or are there some exemptions or other factors involved?

 

One company I'm looking at seems to disclose insider selling in S-8s, while their purchases are disclosed in Form 4s. This results in lots of insider buying activity being visible on websites which aggregate data from Form 4s, without any of the heavy selling shown. However even accounting for this S-8 disclosed selling, some of their Form 4s don't add up.

 

Thanks for any help.