policer vs shaper Tc interval

Can someone please explain what INE means by this when explaining the Policer Tc function

"For an optimal burst size selection, if you are using srTCM to perform traffic admission at the network edge, ensure that your policing burst sizes match at least one packet size more than the burst size configured at the customer side. This is needed because now the router meters traffic with extra precision, and the effect illustrated on the next figure (shaper Bc = policer Bc) may occur."

"Note that the policer may mark the beginning of the second burst as exceeding. This is because the policer Bc exactly equals shaper Bc. In this situation, when the sliding window hits the beginning of the next burst, it has no capacity to accommodate another packet. The packet marked as exceeding does not count for the sliding window content, and thus the next burst will have all four packets admitted. For this ideal simulation, the policer Bc should be larger than traffic burst size by at least one packet. This brings us to the formula:"

They also state "Policer Bc = Shaper Burst Size + Average Packet Size"

I just don't understand why they say the Policer Bc should be bigger than the shaper Bc

If the Shaper Bc refills every Tc, and the Policer Bc refills incrementally then wouldn't they still end up with the same Bc if it matched? Is the reason the Policer Bc behaves like this so that we can have different shaper Tc?