Lip-synching on X-Factor.....

Two "X Factor" performances last night revealed there's clearly lip-synching going down on the talent show ... but a rep says they're not alone, and it happens more often than you think.

During the opening song -- performed by all the contestants -- one singer started moving his mouth way after you could hear his voice. Later in the show ... the Stereo Hogzz backup singers didn't raise their mics at all. Oops.

An "X Factor" rep tells TMZ the vocals on all ensemble performances (not individual ones) are "pre-recorded" ... in order to give the contestants more time to work on competition songs.

The rep added, "This is also no different to what other competition shows do for ensemble performances."

The rep did not mention any of those "other" shows by name.