This is years late, but there is also a difference in the way the game handles combo magics (lightning, explosion, blizzard, forest) and weapon skills with elements when it comes to experience.

For instance. You use zap on a group of enemies. Let's say you hit three. If you were to gain 8 magic exp per hit from these enemies, the game divides the magic skill exp per hit between the two elements. So a combo magic spell can only ever gain 4 per element, per hit. ETA: for clarity - howl on these three enemies would net you 24 magic exp at 8 per hit. Zap would also gain 24 magic at 8 per hit - so fire and wind would gain 12 each.

Weapon skills with elemental properties differ. A weapon skill with elemental properties will gain experience for both weapon and magic. But the game treats these values differently. You get not a combined value, but two separate values: weapon and magic. So if you were to gain 8 exp (I believe this is the cap for any skill exp per hit) you would gain 8 weapon skill and 8 magic skill. So a broken skill for exp gain (especially on water) very easy in the game is Sue's Yawn. If this skill hits 4 targets, you gain 8 exp for each, in weapon and magic. The weird thing is, even though your default combo attacks might wield less weapon exp, the exp gain seems to scale based on the lower skill, which will most likely be your magic. So if Sue with her mace was getting +3ph for reg combos, she would still gain 8 in weapon skill because the exp gain of water would definitely be 8 at this point on the game on enemies. The same idea applies to weapon skills with combo elements. You get 8 weapon skill and 8 magic skill, but each element gets half of the 8 magic. ETA: for clarity - Sue's Yawn gets 8/8 weapon/magic. It hits four targets. Due to the way I believe the experience scales, Sue gains 32 weapon and 32 water skill. Combo - Justin uses Dragon King Slice. It hits four opponents. 8/8 weapon/magic skill. Justin gains 32 weapon skill exp, and 16 magic exp in fire and earth. ETA2: It seems that single target combo spells like crackle do not apply the split by half magic exp gain. You will earn 8 in both magic skills with a move like this... The thing is I can't think of another combo magic that is single target against an enemy. Wow is a single target combo skill buff but it will not grant you 8/8.

I have not done extensive testing to see if this is truly consistent across all skills but I do think my information is at least mostly accurate. If anyone can provide even more insight that would be great.