All the inaccuracies of the Walking With series are the result of Nigel’s unknowingly altering the timeline
Nigel is not really the best time traveler. He often interacts with the local fauna without care and leaves modern day objects behind without bothering to retrieve them (the chainmail he uses with the Dunkleosteus in Sea Monsters being a prime example). As such, any of his actions triggers several butterfly effects that change prehistory as we know it, explaining things the programs of the Walking With series got wrong such as creatures displaying a different size, behavior, appearance or location to their “canon” one.- This also adds up with Walking with Beasts having the fewest inaccuracies, since the Cenozoic seems to be the period where Nigel spends some of the least time in his specials (to be fair, he doesn't spend much more time in the Paleozoic either, but that being such an early and critical stage for vertebrate evolution still means that any small change in the timeline could possibly have more drastic consequences).
- Adding to this even further, Prehistoric Planet represents the natural state of the Walking with prehistory before Nigel started fucking with it. Nigel's first trip back in time would've had him interacting with these more accurate versions of the animals, but he doesn't remember that trip ever happening because, of course, it didn't ever happen after Nigel erased it from the timeline.
The Giganotosaurus of Land of Giants are actually Mapusaurus In-Universe
Nigel merely misidentified them as Giganotosaurus because the species and Mapusaurus are extremely similar and Mapusaurus wasn’t discovered at the date he time traveled to the Cretaceous Argentina, thus he was applying the knowledge he had at the time.