Most likely it's the only obvious answer: not all the storylines cover exactly the same timeline, and for instance he might have to go back in time in Logain's storyline, in the BT storyline, in Lan's storyline, in Loial's storyline etc. We already know this, he spoke of perhaps including some of what happened to Pevara and co. and he said he might include stuff about Danielle/Teslyn's plans for the BT, etc. If he doesn't go back in time some, then he will have to cover all of Loial's backstory with dialogue or POV reflections. Same for Lan - we'd likely catch up with him in the middle of battle if Brandon doesn't go back and cover some of what happened to him between his last scene and the start of the invasion from Tarwyn Gap.

This means in the main storyline (the one starting with the Fields of Merrilor where all the main cast was converging at the end of ToM) the timeline covered from there to the end of the book will be of X months (or more, or less), but in some minor storylines it will be longer, overlapping varying bits of the ToM timelines.

He basically dodged the question, he didn't want to answer how many months/days/years the last book cover, so he rather said it varied depending which area of the world the characters were in. But this was essentially a RAFO.

Of course this immediately sparked many time travel or TAR theories (people forget in TAR it's only perception of time that's different because time passing is how each dreamer perceives it's passing, this has no bearing on "real" time), but Brandon would not have answered that way if that was he had in mind.