The "worthiness" of Mjolnir is something that sort of blurs that magic/science line in how Asgardians are treated in the MCU. (As noted earlier, they're more "aliens" than they are "gods.")
But, it is also a universe where magic exists (see: Dr. Strange, Scarlett Witch's powers, Quicksilver's powers and likely other powers granted by Loki's staff) so that's all it likely boils down to. Mjolnir "works" via magic.
It's likely not heavy in the way we think about it which is why it doesn't crush or destroy objects when its placed on them or hung from them, that or the enchantment on it makes the hammer heavy only when being wielded (for the sake of having more mass and energy for bigger impacts) but when at rest it's light enough to not destroy objects it's placed on. (Thor could, for example, "pin" a normal human with Mjolnir like he did with Loki at the end of "Thor.")
What is "worthy" is obviously variable or could be circumstantial and what actually makes one worthy or unworthy is just the enchantment's "decision." Maybe Odin's idea of "worthiness" when he put the enchantment on it.
Obviously it has nothing to do with physical strength since Hulk couldn't lift it and Iron Man couldn't lift it with repulsor rockets and him and Rhody combined couldn't lift it with repulsors. Cap could nudge it. Either because he's slightly worthy or because he's worthy but the hammer still possess enough weight (even if it may not apply all of it when at rest) that it couldn't be physically lifted by Cap and his worthiness. Which would mean Mjolnir weighs quite a bit since, as noted, Cap is quite strong to super-human levels.
Or it could be at that moment Cap's worthiness wasn't enough to fully lift it, it was a nod of respect from Mjolnir while also being a, "Nice effort, buddy." I suspect that at somepoint in the movies, likely in the Infinity War movies, we'll so see Cap fully wielding Mjolnir. It seems like too much of a thing to put in there to have it not pay off later since it seems a lot of what's happened and seen in these movies all pay off.
Many could complain or gripe about how in the previous movies the collateral damage was shrugged off but here it culminated into something bigger that's impacted the overall arcing story.
So, Cap's nudging Mjolnir is going to pay off. (I'd almost argue Black Widow's refusal to try will pay off too and we'll see her lift it as well.) Mostly with the Mjolnir stuff in that scene with Cap and Widow I'm going to call a "Chekhov's Gun" notion in the overall story in the MCU.)
