Per the ARRL:
A band plan refers to a voluntary division of a band to avoid
interference between incompatible modes.
The regulatory bodies for other countries have similar declarations for their band plans, and there are varying levels of enforcement of those plans. Even countries like the US where the plans and modes are more strictly enforced, the plans themselves are not regulatory. By and large, enforcement is by agreement among Amateurs and a general spirit of conviviality.
A band plan is necessarily a living document that changes over time. Since repeaters are the purview of local clubs and radio organizations, they operate within local guidelines and recommendations at the time they are setup by the community and interested parties. This means that as the plan changes some established repeaters are going to be "grandfathered" in.
Look at the history of this repeater. My guess is that the club that registered it has been around for decades, and either the current band plan was different or it was ignored for some reason and all parties agreed to it at the time. Maybe some local geography made interference with other services or users a problem when the "correct" frequencies were used. Maybe, regardless of the plan, repeaters in this area always used this area of the band. Habit and local custom will often override other factors when making decisions by committee!
But the main takeaway is that a band plan is voluntary arrangement to be used as a set of guidelines to make it easy to share that part of the Amateur spectrum, and not necessarily a legal or regulatory agreement.
Since all Amateurs are allowed equal access to the bands they are licensed to use, you have to negotiate with all Amateurs, including the maintainers and users of any repeater(s), for how to share any of the Amateur spectrum. In this case, "good operating procedures" (since moving repeater frequencies isn't a trivial operation) suggests that the best way to handle it is to move (for 2m, assuming some sort of FM) 10-30 kHz up or down the band and try simplex there.
The clubs in your area would probably know about any other typical traffic around frequencies of interest. For all you know, there is a monthly net that has been running for years on 146.49 MHz. Anyone could be using any frequency you choose for this transceiver at any time in the future. You don't own any frequency even if tuning is a hassle. You are negotiating to use the frequency at your bandwidth every time you send power to the antenna.
It may be advisable to pause on retuning this equipment and simply spend a few weeks listening and talking to local hams to see what other kinds of traffic to expect around your frequency of interest. Amateurs are usually accommodating, but you can imagine folks not being too happy if you stepped on their net from out of nowhere with a claim that you are on equipment with a fixed frequency and they have to move.
There is a reason fixed frequency devices like repeaters go through a local committee before being fired up. You may need to treat this transceiver like a repeater and coordinate with your local clubs to make sure everyone is on board.
Per the FCC regulations Title 47, 97.101(b) since you mention the ARRL (emphasis mine):
Each station licensee and each control operator must cooperate in
selecting transmitting channels and in making the most effective use
of the amateur service frequencies. No frequency will be assigned for
the exclusive use of any station.
Plainly put: assuming you have exclusive access to the spectrum around 146.46 MHz when you fire up your equipment is probably not going to be received well by most Amateurs, is contrary to the spirit of fair and equal use of the spectrum, and a band plan is no remedy for this. Even repeaters don't own those frequencies. They've just been given primary use by local frequency coordinators.
(I am assuming here that you are observing the actual regulations, and using equipment that is essentially fixed to a single frequency is allowed under the regs where you live.)