My ControlNet network shows a maximum usage value of 99.61% when an MVI56-XXX or MVI56E-XXX is in the rack, why is this?

99.61% represents the allowable ControlNet bandwidth. The bandwidth rule is that each scheduled node gets 1 and only 1 opportunity to talk during the scheduled portion of a NUT (Network Update Time). The maximum amount of data that any node can transmit at any time is approx 500 bytes. So anytime a node transmits 500 bytes the network will be at about 99% usage. Since the MVI56/MVI56E modules send 250 INT values (500 bytes) , it is guaranteed that if the module talks the maximum bandwidth used will be 99%.  Therefore it is normal to see the 99.61% for maximum usage when using a standard ProSoft MVI56/MVI56E module in a remote I/O rack over ControlNet (ProSoft does make R (reduced I/O size) variations of many modules that utilizes a smaller I/O size on Control Net).

Some other considerations with ControlNet:

The normal usage value for the Control Net network is the average bandwidth over a 128 NUT cycle. If using a 1756-CNB keep in mind that this module is somewhat CPU power limited and can have issues if it is at 100% peak and average so RA recommends that you keep the average in the 50-70% range. A 1756-CN2/B has plenty of CPU power so it could be at 99% peak and average with no issues (other than you cannot add anything else to that system).

Keep in mind for ControlNet communications to work the network must schedule and to schedule you must be under 100%. But as stated above a 1756-CNB can have issues as the average gets bigger but unless you try your exact system you will not know what the 1756-CNB can tolerate. Remember that a 1756-CN2/B has plenty of CPU power so it is highly unlikely to ever have any issue related to peak and/or average bandwidth.
The test is that after you schedule your network you check the 1756-CNB CPU% which can be done in RSLinx/RSWho and Rockwell recommends that the CPU% of the 1756-CNB be under 70%.  For the 1756-CN2 this can run close to 100%.