It is possible witnesses lie - the trial is about whether the prosecution proved the offence beyond reasonable doubt. If the jury have a reasonable doubt about whether the girl was too intoxicated to consent or a doubt about whether Ched Evans unreasonably believed he had her consent, they will return a verdict of not guilty.
The question for the jury is how sure are they that she was too intoxicated to consent and how sure are they that Ched Evans' belief that she was consenting to sex with him was an unreasonable belief - she says she had a blackout so she can't remember the night, and thinks her drink might have been spiked.
There is video evidence of her squatting down in heels to pick up a pizza - so she could balance and squat and walk when she went into the hotel room.
Two other men who had sex with her around the same period as Ched Evans had sex with her described her saying very similar things during sex that Ched Evans reported she said during sex with him, that led to his belief that she was consenting. One of the witnesses in the retrial said he had gone to the police during the initial investigation but that he had been questioned about her loss of memory after sex, not about what she said during sex. The witnesses, Ched Evans and Clayton McDonald could all be lying and have somehow communicated before being questioned by the police or before the retrial and agreed their story about what she said during sex in order to make it seem like she was not too intoxicated, was aware of what was happening and consented - it's possible. So I am NOT saying that it is NOT POSSIBLE that she was raped - but it does create reasonable doubt IMO, which means the prosecution haven't proved Ched Evans raped her beyond reasonable doubt.
This is why rape cases are so difficult to prove when it is unclear if someone had sex consensually or not. It's not like being mugged - getting assaulted and having valuables taken is not something people sometimes consent to and at other times don't, depending on the person assaulting them. If there is evidence that someone dishonestly took goods belonging to someone else - you have the basis for a conviction. If there is a reasonable doubt they may not be convicted.
But being mugged or robbed is a lot less intimately invasive than rape, hence people find it harder to accept that if there is reasonable doubt the accused, who might be a rapist, is found not guilty. Especially when idiots then claim this proves the woman was lying and start abusing her. Especially in this case, where she has not accused anyone of rape, but just kept saying she can't remember what happened.