They all have differing methods of measuring sales. Some look only at independent book stores. Some look at all print sales. Some look at Nielsen Bookscan data.
When indies claim to be a bestseller, it's usually on Amazon. While I applaud anyone who hits the top lists, quite often even here the term bestseller is stretched in that:· Anyone can hit 'Bestseller' for a very short time frame through flash mobbing (because Amazon focus so strongly on 24 hours worth of sales).
· Some authors consider #1 in genre or sub genre to be a bestseller... which they are, in that subgenre.
· #1 isn't global - it's easier to hit #1 within a smaller geographic area.
It's aimed at a very small niche who presumably aren't buying much when they're in the middle of exams/ finishing for summer holidays. Half a dozen sales in one day is all it takes - less than a tenner can buy this type of #1 spot.
So, next time you see '#1 Bestselling author' in a twitter profile, ask them how they define it.