Correct (original) description of these item behaviors was posted on old-old forum, but it's lost now, old-old forum is no more.
I think it was something like this. Default behavior is just that, default, you have item, you use it, it does it's thing and that's it. Other two are Affects all girls temporarily and Affects one girl temporarily. This temp effect was meant to last for 20 turns while diminishing every turn, to get to 0 at the end of these 20 turns. That makes it somewhat easy to not screw up original stats/skills. Simply add, let's say, 40 to that stat/skill and then subtract 2 points each turn for 20 turns. It would return to original value. And if that exact skill/stat is improved via some other way (like training) it won't matter, as this subtraction is set, and won't subtract more than 2x20 points.
Things get somewhat complicated if this temp item gets used twice so there's like another 40 points added 2 turns after first 40 points, at code side at least. But it could be made to work, even if it means making it so that girl can be under effect of only one temp item at a time so there is one thing less to worry about.
Although there could be conflicts here, trait management being the largest, if girl is given one trait via temp item will it be deleted after 20 turns even if she uses another item that gives her this trait permanently, and similar. One another potential problem could be max values for skills/stats. If girl has it at 80, and gets under effect of item that raises it for 40 how will that turn out? 100 is max, will she have 60 after 20 turns or will it decrease to correct 80.
Story wise temp items could have their use, like making some items that are otherwise very rare commoner in their temp form. Like cure for some diseases or addictions. If you can't have the real thing temp one would be good enough until permanent one is acquired. And this just gave me an idea how to "fix" trait conflict, and perhaps others also. If trait/skill/stat is under temp effect and permanent one is used, temp one is canceled and permanent gets into full effect. For traits this would be somewhat easy to do, it's simply on or off, for skills/stats this will be somewhat harder, it would probably mean that original (base) value would need to be stored somewhere, and probably added to if that stat/skill would get changed via training or like.