@Stotter
A point to that, one messed up future (which lead to this screncap) was twilight going too angry and crystalforming RD all because Starlight dodged.
@Dragonmage
My guess: Same reason this scene happened. She had to fight while at the same time not hit so much as a butterfly for fear of a new temporal effect. A blast strong enough to knock out Starlight would have ended a little pegasus. Also the fight itself could/did become enough of a spectacle to mess with time. In short, the battlefield was too fragile for a proper fight. Starlight just had to not lose.
@Background Pony #073A
Exactly, she outwitted her foe, and not just in the fight, but the situation by tying twilight to herself, allowing her to mess up the past before twilight could stop it each and every time.
@Daneasaur
Honestly, you people didn’t really watch these fights, did you? Sorry, that came off a bit too confrontational. =P
Anyway, Starlight spent nearly all of those fights dodging and countering. She didn’t beat Twilight in magical ability, she beat her in combat prowess. She allowed Twilight to exhaust herself, mostly just dodging every single one of her attacks.
@Dragonmage
one of the many unanswered questions about her.
However, this is touched on by twilight. Twilight, who has a mark showing her power over magic, isn’t nearly as powerful as her. Starlight, who found a way to ENHANCE Starswirl’s spells.
Who would be so arrogant to think Twilight was the be all end all of magic? How about a pony who has vastly more potential in magic than Twilight? Someone who trained herself and honed her ability to manipulate the fabric of all magics due to an obsessive grudge?
Also, Celestia never taught twilight in the ways of combat via magic, at least, not that we know.