The way the models were released doesn't really support that. The first model that had that crewman was the current land raider. When it came out, white dwarf did an article said that all land raiders had two normal crew like any other vehicle and besides those two they always had a techmarine on board. So you can assume that the crewman model who came with that land raider model is that techmarine.
Then, they made the current rhino. You'll notice that the techmarine is on the same sprue as the storm bolter, the radar dish, the HK missile, the hatches, etc. They just reused the sprue from the land raider, because it is easier than making a separate sprue. So model-wise, the techmarine is just optional the same way that the HK missile is optional. Fluff-wise, the Land Raider always has a techmarine somewhere, but a rhino or predator doesn't have that requirement. The storm talon hasn't ever been said to have that requirment either.
Then, yeah, Phoenix01 posted the thing that he did.
"In some cases, it will be the Master of the Fleet's own company who provide these Marines, with each of his veteran captains acting as captain to a different vessel within the fleet while their own squad members each man a different vital area within that same vessel. In other cases, squads from different companies within the Chapter may be charged with manning the fleet, serving under the command of the Master of the Fleet in just the same way as a Space Marine battleforce may be made up of squads drawn from several companies across the Chapter under the battle-command of a single, nominated force commander."