There's only three things I an consider happening here. Either one or the other is occurring, or some combination...
1: The damage dealt by an upright spear/spike depends on the distance dropped, as it effectively takes your falling damage and applies that as the power of the weapon's attack. Falling from higher up increases the damage the spear does, which alters the way experience and skillgain is handled. I'm aware that skillgain doesn't depend on difficulty of task, so...
2: Toady may have slipped a variable somewhere, and accidentally referenced the wrong item. It may not be a fluke of design, but rather an outright misplaced value, where it's trying to assign the damage as experience. Because of the weird way combat is handled, dealing damage in the tens of thousands might only translate into bruises, but if those tends of thousands are accidentally put in the 'experience' column, you get supersoliders.
3: Weird conversions are happening. It's possible that falling damage is figured differently from weapon damage, because normal weapons are made of metal and wielded, while the ground is made of relatively soft 'floor' and thus needs highly exaggerated damage values - an axedwarf with a copper axe might deal 30 strength through 10 hardness, for 300 damage, but a floor tile only has a hardness of .1 so it needs to hit with a strength of 3,000 to deal any reasonable damage. That 3,000 strength strike is only a gentle pap, but it's somehow applied to the upright spear as well, and from there somehow works its way into the experience given...
Whichever way you look at it, this is a very deep coding issue, not something that can be explained via any in-game means.
In any case, I think it'd tag quite nicely on the Children of Armok project...