It's just an eBay thing.

People sesrch by terms and most people interested in our scale will search using "1/1250" because that has become the dominant scale, even if they are looking for models by producers who work in 1/1200.

Sellers know this and will often add 1/1250 to the title and description to get the greatest number of views, even when selling LJ or B-L, which are always 1/1200.

It's not usually done to deceive as when resin or metal models come out of the oven or whatever and cooled down the difference between scales has often become blurred anyway.

I learned long ago not to measure the length of my favourite models, just enjoy them for what they are!