According to Winston, why is it possible that the "truth may be a lie?"

The truth, according to the Party is what they deem to be the truth. History is rewritten...... truth is only truth for as long as the previous truth's memory still exists.

And if all others accepted the lie which the Party imposed— if all records told the same tale—then the lie passed into history and became truth. ‘Who controls the past,’ ran the Party slogan, ‘controls the future: who controls the present controls the past.’ And yet the past, though of its nature alterable, never had been altered. Whatever was true now was true from everlasting to everlasting. It was quite simple. All that was needed was an unending series of victories over your own memory. 'Reality control', they called it: in Newspeak, 'double-think'.