It's kind of funny. My young teenage daughter plays in our group and when we discussed the common item concept and I mentioned that only about 1 item in 30 is common with the current definition, she responded with "1 in 30 items are common? How's that common? I don't think that word means what WotC thinks it means.".
Sometimes kids see what adults do not.
Actually, it makes PERFECT sense.
Say I'm a magic blacksmith. You bring me a load of Rubidium, and want me to make you a suit of armor. I know how to make plain, basic +1 Rubidium Armor, which is easy and I can knock our quickly. Any other blacksmith can also make the same +1 Rubidium Armor.
On the other hand, if I'm feeling particularly adventurous today, I could spend five times as long lovingly crafting a suit of +1 Pulchritudinous Rubidium Armor, into which I weave a spell that grants the wearer gorgeous, wavy blond hair. This spell and this armor are my own creations, and are unique in all the world. If another blacksmith wants to make it, he'll need to figure out the spells from scratch, and will likely end up with a different result than mine. After all, Pulchritudinous Armor is more a work of art than a simple object to be constructed.
I have other options as well; I could craft +1 Armor of Stinging Nettles, +1 Armor of Badgers, or even +1 Windsurfing Armor. Like the Pulchritudinous Armor, these works of art would be wholly unique.
Magewright Wilkins down the street can also make armor from Rubidium, but you'll get different products if you shop with him. He can make you your generic +1 Rubidium Armor, but his skilled hand might also whip up a +1 Armor of the Glaring Tapir, or the dreaded +1 Armor of Pandemic Herpes. Like my work, Wilkins' creations are unique, and can't be replicated by my hand.
And so on, and so forth. In the end, you're left with a very long list of things that could be made with your Rubidium. At the top of the list is the plain +1 Rubidium Armor; it's the easiest thing to make, so you'll find it absolutely all over the place, but in the end it's still just one item on the list. As you progress down into the more difficult items to create, they start to take up more slots on the list; because while there are less Uncommon suits of armor out there in the world than generic suits of +1 Rubidium, they exist in a far wider variety.
Or, in other words: your daughter should probably take an economics course.
