No. I mean, strictly speaking, one of those things is true.
Successive gold multiplier runs on level 17 beasts with no team bonuses. Both give 100% of the expected gold in each run. XP for the 74% completion is about 80% the xp of the 100% completion.
- 74% completion is enough to get full gold (in at least some cases)
- 74% completion is not enough to get full XP in at least some cases
Having heard this max-XP-at-70 legend before, I suspect it depends on level. Max XP increases more frequently than invader tower level, so at some levels your invader’s towers must represent more than enough XP, and you should be able to get max without killing them all.
I would also believe faulty memories or a bug/implementation inconsistency. (This is War Dragons. Bugs have to be high on the explanation stakes for any phenomenon.)