Rapid battery drop MBA

Hello,

i would like to ask about battery problem on macbook air, since i bought him i had it connected to the cord like normal pc, time of time i took it from charging and i was holding 98% 97% battery capacity.

ONE DAY i used the notebook just on battery because i was traveling and the battery capacity dropped to 91%, is this some sort of joke, or this is how apple forcing clients to buy or stop buying their products?