Having decided to terminate your contract, they probably took a look at your holiday plans and decided to implement termination at the end of that period. When you say you are not required to work the last week, what your employer is probably meaning is that since you have booked this time as holiday they obviously don't expect you in work. I don't believe you have an entitlement to change this plan now, in much the same way as you wouldn't expect an employer to cancel your holiday plan once booked. It works both ways.
Regarding how much holiday entitlement you have depends on the company policy. Your figures above suggest the company gives 22 days holiday per year, you maintain you have done 2 months work implying 3.7 days (your calculation, I'm assuming). Once again this depends on company policy. Most companies I've dealt with calculate this on a weekly basis then round up or down to the nearest half a day - not monthly. You will have to ask them. Don't forget that the entitlement may include public holidays, again according to company policy, and there's one of those in your final contractual week of employment - next Monday. Maybe that's where the discrepancy occurs.