User blog comment:FairyRaven/One-time-only Spells/@comment-904888-20120709005643

Lucy's Uranometria: I agree, it does seem weird that no one is surprised that she, as a celestial mage, could cast something besides Celestial spirits (which no other Celestial mage we know to date are capable of). But the means I am not too surprised. Remember that she had to summon Gemini, which have the ability to copy another Mage along with his/her abilities, and they have the ability to remember their last 2 transformations. It is entirely possible that they copied the version of Lucy that KNEW Uranometria, and the CURRENT Lucy was just a mean to give the Gemini Lucy enough MP to cast it. IE Gemini was the one casting it, rather than Lucy (at least this is what I think). If Lucy knew Uranometria herself, she unlikely would have needed to summon anything to cast it.

Fairy Glitter: This is only my theory, but I think Fairy Glitter is the single target (possibly non-comrade discriminant) version of Fairy Law. If one thinks Law to be a Nuclear Missile, then Glitter could be a tactical nuke. Having Fairy Glitter being more powerful than the spell cast by Jura (a Wizard Saint) tells us nothing about the nature of mages, but rather the spell themselves. I have no doubt that Jura's power, as a Mage, would be able to trump Cana.

The reason why Glitter failed against Bluenote, but succeed against MPF, could be a variety of factors: 1. It was her first time casting it against Bluenote, so probably inexperience with the spell took over. 2. Remember she trained for 3 months like everyone else in FT (Except those who 'fast tracked' courtesy of Ultear), so her abilities would have become more suited to cast FG 3. MPF is an inanimate object, with no ability to interrupt FG. Bluenote is a mage of Gildarts caliber, and he was able to prevent FG from casting successfully. 4. With Cana's friends in danger against Bluenote, she wanted to get FG casted as soon as possible due to the sense of urgency. That urgency did not exist against MPF, there wasn't anything that threatened FT, so Cana could have calmly casted FG, ensuring it is properly and successfully cast. 5. This could be controversial, but Cana was DUI against MPF, but was sober against Bluenote. Possibly hinting that she is more powerful while drunk?

In any case, power of spells themselves do not quite give an accurate indication of a Mage's ability, especially if it is just the one spell. This is most evident in Jose vs Makarov fight, without using Fairy Law, the two saints were evenly matched.