Well, the "plastic sleeve" is snug ...
The "plastic grommets" (which also acts as part of the weather seal) which reduces the Inner Diameter to 24mm are the same size for both cups ...
The only difference (besides the threads on the cups, of course) is the meager amount of stenciled information on the outside of the cups ...
But, the plastic grommet is not so snug that you shouldn't be able to tap the spindle into the cup(s) with the heel of your palm or side of your hand ...
[CENTER]------[/CENTER]
HMmmm ...
If you can post a picture of your crank's spindle, that might help clarify what you are looking at ...
Generally (that is, in the past), the spindle on Shimano Hollowtech II & FSA MegaExo cranks are bare "silver" where it contacts the plastic sleeve AND a dull, flat finish in the middle of the spindle (presumably,, a rust inhibiting coating ... it is NOT threaded) ... and, the end is splined to mate with the non-driveside arm.