I think I can explain why you saw a more expensive ticket from Shelford <-> Tottenham Hale, than for Shelford <-> London Terminals. (My copy of Avantix is a little out-of-date so I can't match exactly the fares you're seeing.)

An 'any permitted' ticket from Shelford to Tottenham Hale is valid via the following routes:
Shelford direct to Tottenham Hale (going through Bishop's Stortford, Harlow etc)
Shelford - Cambridge - King's Cross (or Moorgate) - Liverpool St (?) - Tottenham Hale (? because I'm not sure whether you can just go straight to Tottenham Hale rather than via Liverpool St)

There is also a 'Not London' Shelford to Tottenham Hale, which is only valid via the first of those routes.

A Shelford to London Terminals ticket 'any permitted' is also valid both ways, but as soon as you reach a London Terminal, you have to stop.

So in theory, the 'any permitted' Shelford to Tottenham Hale allows for some cross London travel, and is hence more expensive.

The rules however are STUPIDLY COMPLICATED and I may have entirely misinterpreted them.