Both sellers have the buy box?

I’ve noticed this on several listings this month. When I check my phone, seller A will have the buy box- but I hop on my computer and sure enough it’s seller B in the buy box. Another way I’ve noticed this was even though another seller had the buy box (or at least appeared to on my computer), I was getting multiple sales. I usually never get sales when i’m not displayed in the buy box. Just wondered if this was common, i’ve been selling for over a year now and I am just now seeing this.