When does 97 = 100?
When Google inserts results for a related phrase, apparently. If you check the 2nd page for the original term (I know, who ever does THAT, eh?), the numbering starts from 98. Therefore, Google are definitely actually inserting data from a related term into the SERPs when you see that behaviour, with the implication being that both result sets are being retrieved and amalgamated before being served to the user.
Since this behaviour is only ever really seen on somewhat ambiguous, and in my experience slightly obscure terms, I wonder if it’s a way of indirectly determining what terms Google perform badly for according to their own internal metrics (CTR from SERP, follow-on searches etc), since the positioning of the insertion is variable (ooer, missus!).