My understanding is that N represents the largest number of bicycles owned at one time: I assume that N can increase, but can never decrease. Let t1, t2, and t3 represent three successive points in time, and let x represent an arbitrary but stable number of bicycles. Hence:

t1: x bicycles are owned. N = x; N +1 = x + 1. You have N bicycles.
t2: 3 additional bicycles are acquired. N = x + 3; N +1 = x + 4. You still have N bicycles.
t3: 3 bicycles are sold. N = x + 3, since N cannot decrease. N + 1 remains x + 4, for similar reasons. You now have N-3 bicycles.

So basically: at t1, the correct number of bicycles to own was one more than x, the number that was owned at that time. Now, the correct number of bicycles to own has increased to 4 more than you currently own.

Now, this can be avoided by discharging bicycles prior to acquiring new ones. After all, N is based upon the largest number owned at one time. Were one to discharge three bicycles, N would remain the same, and one would be at N-3. Acquiring three new ones would put one back up to N.