It’s to do with how HMRC regard the value of the bike at the point that ownership transfers.
Employers used to just transfer ownership for free (or a small admin fee) at the end of the 12 months, but you can’t really argue that a year-old bike is worth nothing. That then has tax implications (for both the employer and employee) so the easiest solution is to sell at “fair market value” (20-25% or so), or else to pay a one-off amount to go on hiring the bike for 3 years.
Much like a computer and other business things, you can say that a 3-year-old bike is worth nothing, so they can then transfer it to you at the end without it being a tax dodge.
Actually, it still has a residual value until 4-5 years old – https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/457866/cycle-to-work-guidance-update.pdf
So the £63 will include some amount to reflect what the bike is deemed to be worth at 3 years old.