Because both those you mentioned have series connections involved through the lamp. Take the lamp out and you break the entire circuit and remaining installed lamp won't function.If I remember correctly the magnetic bi bins typically needed two lamps to function properly.
The eight foot single push-in's fed the beauty through on of the pins.
The electronic ballasts seem to function okay with one lamp.
I have a question about removing a lamp from a two lamp fixture. Does removing one fluorescent lamp from a two lamp fixture damage the ballast? Magnetic or electronic.
I was investigating the cause of a arcing fault fire started by a fluorescent fixture last year in a 80YO lady's house. I might have made a post on here about it. I traced it back to the 'series' side of the lamps. One of the tomb stones must have gotten to hot or the ballast must have been firing a start up pulse thru weak connection until plastic melted and combustion started.Because both those you mentioned have series connections involved through the lamp. Take the lamp out and you break the entire circuit and remaining installed lamp won't function.
When electronic ballasts came into play, some don't use any series connections, even for same lamp type, some do.
Most T8 lamp types AFAIK were all parallel connected. You can remove any one lamp and the rest still function. You could use a 4 lamp ballast as a replacement for a 3 lamp fixture and just cap leads for the 4th lamp and it worked fine.