Pony POV Series: Different Elements of Magic.

OPTIONAL CANON! 

Each Element of Magic is unique. While each can perform a slight variance of spell copy, each expresses it somewhat differently. 

Twilight Sparkle can replicate the effects of nearly any spell she sees.

Sunset Shimmer can create much more powerful but gas guzzler versions. 

Trixie is just the opposite, creating weaker, but more mana efficent versions. 

Starlight Glimmer can take spells from different, or even opposing schools and combine them, even creating a mental control spell from odds and ends powerful enough to enthrall the mane six. A truly impressive feat indeed worthy of the history books. 

Celestia's is unknown.