This rings bells with me.
Different makes of the same sized tyre can vary considerably in size so where a cover may fit another brand of 265/75x16, it probably won't fit a BFG 265/75x16.
This was first brought home to me about 18-years ago when I was after a semi-rigid cover for my new 265/75x16 BFG AT's. I had picked one up from a local supplier and immediately tried to fit it in the carpark but discovered that there was no-way it was going to be persuaded into place. I went back inside and the first thing the chap on the counter asked was the make of tyre, when I said it was BFG he went into the stockroom and came out with a similar looking cover which had a sticker in the inside stating that it was suitable for BFG tyres. Went back outside and it fitted perfectly.
Roll on about 15-years and I was now using 265/75x16 Cooper ST Maxx, these were even larger than the old BFG's meaning the old cover would no-longer fit and remembering the problems I previously had I took the easy option and just ordered a cover from China off Ebay. Amazing cheap, arrived within a couple of weeks and it fitted perfectly.
A little later I fitted a set of new 265/75x16 BFG AT's KO2 (I disliked the road manners of the Coopers) and found them to be the same size as the Cooper's so the new cover was a direct fit.
Earlier this year I was by now bored with the large Land Rover logo on the cover so ordered a plain one from China. Although a different supplier (still off Ebay), that also arrived very quickly (some UK shops could learn something from these overseas suppliers) and once again was a perfect fit.
Check your measurements and take a risk on buying from overseas, if you ended up being really disappointed (I haven't been) then you always have your buyer protection to fall back on.