Is the total of 173 hours excluding bank holidays ? If there are a couple of bank holidays, when you won't be at work but you'll still be getting paid, that would be about right.
Most bank holidays are on Mondays but you'd normally be at work on Friday 22nd April, Friday 29th April (if your employer is closing the business for the Royal Wedding) and Tuesday 27th December. So the sums might be right if you've only been quoted the number of hours that are left after 'fixing' part of your holiday entitlement.
If you friend normally works on Mondays, he/she will have far more 'fixed holidays', so the number of hours remaining for his/her holiday entitlement will be far fewer than yours.
Chris