How is the gap calculated?
Do they literally select two people with the same jobs and exp and compare their pay?
Of course not, because that would result in no gap and that's not a figure that's useful for the story they want to tell. There have been like for like comparisons and they do return the same pay.
What they usually do is compare everyone working at least a set number of hours per week, with complete disregard for what jobs people are doing, what qualifications they have, how much experience they have and even how many hours they actually work.
And no, I'm not joking about that last part. They really don't factor in number of hours worked. They just take a cut-off point, which can vary from as low as 16 hours per week to as high as 35 hours per week. So, for example, the "create a gender gap for political purposes" figures
never make any distinction between someone working 40 hours per week and someone working 35 hours per week. At the same rate of pay, the person working 35 hours per week gets 87.5% of the pay of the person working 40 hours per week. Voila! A gap!
Most of the "gender gap" is
directly accounted for by the different average number of hours worked by men and by women (as in the 40/35 example above). The rest is plausibly accounted for by the indirect effects of number of hours worked, since higher paid jobs tend to require longer hours and people who work longer hours are more likely to get promoted.
A big factor in the amount of hours worked is parenthood. The average hours worked by mothers is much less than the average hours worked by fathers, which strongly skews the overall averages of men and women. This is probably the main reason why the "gender gap" varies a great deal by age. The older the age, the larger the proportion of women who have children.
None of this is hidden or disputed, but dominant prejudices and propaganda are more influential than the truth so everyone is endlessly fed the lie that women are paid less than men on a like for like basis, i.e. that the gender gap is a sex gap.
There is some scope for truthful examination of possible trends of sexism affecting the gender gap, but that would be far less useful for feminists than lying about it and could become counter-productive for their goal of promoting sexism since addressing the truth about the issue might result in more sexual equality since that's the only fair way to close the gap. Since the goal of feminism in this area is to have women paid more than men for the same work (so the average pay is the same when the average hours worked are not), why would feminists do a U-turn when they're winning?
For people under 30 in the UK, the "gender gap" is the other way around - the average pay for men is lower than that for women in jobs classed as "full time". This, unsurprisingly, is generally ignored. This gap is, again unsurprisingly, increasing rather than decreasing. When preferential treatment and status is routinely given to one sex, it's bound to have an ever-increasing effect on average pay by sex. I might still be alive when the average pay for women is higher than that for men at all ages. It might be mildly entertaining to see how that's spun to portray it as discrimination against women.