Was not the "B = byte, b = bit" standard established even back in the day, in the 1970s/early 1980s?
Sure, it was, but magazines and the like were not only consumer publications, but as well made by only partially educated people. Everyone wrote like he thought it would fit. More so, I don't think any country ever invoked a spelling police for computer magazines, or did they?
While some magazines let each author decide, others tried to evoke their own conventions to keep present consistency to their readers. These conventions were based on different reasoning from spelling (always writing Kb and Mb for bytes), over heritage (like electronics/communication magazines using strictly only decimal prefixes, as was common for transmission rates) and avoiding (always writing in full or as word like Kbyte/Mbyte) to attempts of avoiding collision (like avoiding collision by use 'unused/' SI combinations KB and mB for binary Kbyte and Mbyte).
In the end it differed from publication to publication and country to country. The Wiki entry for Binary Prefixes tries to shed light on some of the origins and attempts to straighten it.
Yes, I also know about the "MiB" stuff, but it never seemed to be used by anyone.
Well, I do, and many others as well. Try it yourself. You'll get used soon and laugh about all the inconsistency others still produce.