Men comit 98% of all sex crimes. Women comit very few, relatively speaking. Therefore even a few crimes recorded as being female but committed by trans identifying males will skew the figures.
The number of biological women in England and Wales prisons for sex crimes is something like 160 out of a prison population of around 4,000 (around 4%).
In 2019, there were 129 transwomen prisoners, of whom 74 were sex offenders - that's 57%. So it looks like transwomen are more likely to comit sex crimes than other crimes. If these were recorded in the female stats it would skew the figures. It would put the female sex offenders figure up by 50%.
This table doesn't include transwomen with a GRC. They are already recorded in the female sex offender stats, I understand.
As not all police are recording by sex anymore it's getting harder to get accurate statistics in other areas like number of arrests or charged.
Accepting transwomen as women in law, whether with a GRC or not, normalises allowing men into female single sex spaces. No, you don't need a GRC to rape a woman or girl but the whole notion of self-ID definitely helps.