They roll it over but it is the last used and is only rolled over a single month.

Month 1: Get 10 Use 5 = 5 rolled over.

Month 2: Get 10 Rollover 5 Use 5 (of the 10) Lose 5 (already rolled over) = 5 rolled over.

Month 3: Get 10 Rollover 5 Use 5 (of the 10) Lose 5 (already rolled over) = 5 rolled over.

Month 4: Get 10 Rollover 5 Use 5 (of the 10) Lose 5 (already rolled over) = 5 rolled over.

By Month 5, you would think you would have 30 (10 + 5 rolled over from each month) but you only get 15.