Weight loss always involves some loss of lean tissue in addition to fat. Dieters can mitigate this to an extent by protein intake high and maintaining an exercise program.
And sleep. It's rarely mentioned, but the difference between good and bad sleep may mean the difference of 80/20 fat/muscle loss, and 20/80. If the numbers are true (I know of at least one study, but no meta stuff) it means that crash dieting with bad sleep is actually counter productive, as you lose a lot more muscle than fat.