First thing I would check is for voltage drops across all connections between battery, alternator, and ground. No connection should produce more than .1 (one tenth) of a volt drop. Ideally voltage should be the same everywhere in the circuit.
If that checks OK, I would think that the battery is degrading, Your voltage reading is dependent on the load placed on the alternator. In an older battery, repeated charging and discharging produces a coating on the plates, and this reduces the ability to accept a charge. It becomes more of a Capacitor than a power/load source. Also AGM batteries have different charging needs.
I am not familiar with your alternator setup, single wire or second lead to regulator? If single wire, probably no problem. If it has second wire, then again, connections are important.
A 90 amp alternator is a large output value for a motorcycle battery, but that amperage is only realized as loads from accessories, lights, and ignition are placed in the circuit.
After starting, the battery is briefly in a discharged state. At that point, it's voltage is lower than the alternator voltage. It is presenting a "load", once the battery voltage equals the alternator voltage, and that voltage is equal to the regulated voltage, all charging should taper off. 14.2 volts is all that is needed to satisfy a healthy battery. Very small resistances in the charging circuit can cause the "load" to be false, allowing the voltage to rise.
Before the days of electronics, some mechanics would start the engine, then disconnect the battery, if the engine died, then they assumed the alternator was bad. This is not a good thing for electronic circuits because if the alternator is OK, it's output voltage will run "wild" because of lack of load from that battery. Using the electronic circuits a load, for voltages >16 volts.
Does your motorcycle have a tendency to drain the battery when not running? I ask this because you said you have a battery tender. Some are good, others only amount to a 1 amp continuous charger, with no regulation. This promotes the "plate coating" I spoke of earlier. A proper tender should be active, shut off at a given voltage, turn on a given voltage. 12.7-14.2.