Given the phenomenally long operational lifespan of a BattleMech if nothing actively destroys it, there would be a lot of older equipment still in service. You can upgrade some of it over time, but in a lot of cases it's more than adequate for the task it's assigned to. Upgrading isn't a high priority, if it's even authorized. I can picture a lot of basic tech 'Mechs being operated in remote places not near an active border, but still in need of a garrison to deal with potential revolts or pirate raids. Then the whole Star League went pear-shaped and they were called into front line service, because yet another part of the old front line (and the city it was stationed near) had suddenly ceased to exist.
Some units may have been intentional downgrades, for legal sale to the surrounding House armies. The factories would likely have been required to remove some exotic piece of equipment which was not permitted for sale, and replace it with some other item or combination of more standard items to fill the space. Guess what? The design is now compromised and way less than optimal, but still reasonably effective at its intended purpose. The only thing I see contradicting that is how mediocre a lot of the 2750 Star League designs ended up being, even with that exotic equipment.