It's a pretty dumb ordnance, gravity delivered GBU57 is a physics bound problem. The dimensions etc are known, you can give it the most optmistic assumptions, i.e. complete steel for max penetration, release at altitude where it reach max terminal velocity without grid fins deployed, run that through ndrc/young pentration equations etc. There aren't any super secret parameters for subterfuge like electronic warfare. Eitherway there's public videos of GBU57 in action - grid fins deployed to hit a traffic cone - defense autists counted frames, did napkin math, it's more or less what's purported ~ mach 0.8-1.2 penetrator designed for ~60m concrete. IIRC the assume sphere cow math for heavier all steel, no grid fin (i.e. not accurate), max out at mach ~2, doubles energy, penetrates ~80m.
On the other hand, Fordow's construction time is known... as far as I know, many years before fgcc / uhpc and other "advanced" concrete formulas PRC formulated against US penetrators. And Israel probably has entire blue print, so who knows. E: quick lookup and GBU57 seems to be revealed shortly after guestimate of when Fordow started construction, possible Fordow could update design in anticipation, but then again, B2s were known entity and Iran's engineers can probably guestimate out what the maximum size/weight penetrator US could deliver on B2s before knowing GBU57 existed.
What if it has some sort of a booster to increase its kinetic energy just before the hit?
Also the behavior might improve in an area already weakened by a ventilation shaft/previous hit (first bomb turns 40 meters into fine gravel + detonates weakening quite a large are, second and third bomb easily go deeper)
I think 1) is unlikely, b2 bays can't fit much more, gbu57 is mostly metal and no booster for penetration 2) is what no one knows, but we (as in the public) also don't know layout/construction, i.e. actual depth, bunker design (can emb sloped concrete/steel layer to deflect penetrators laterally so follow up drop don't go straight down).
On the other hand, Fordow's construction time is known... as far as I know, many years before fgcc / uhpc and other "advanced" concrete formulas PRC formulated against US penetrators. And Israel probably has entire blue print, so who knows. E: quick lookup and GBU57 seems to be revealed shortly after guestimate of when Fordow started construction, possible Fordow could update design in anticipation, but then again, B2s were known entity and Iran's engineers can probably guestimate out what the maximum size/weight penetrator US could deliver on B2s before knowing GBU57 existed.