I'd take dev quotes with a pinch of salt, PIPs to shields makes a big difference if you are doing the interdiction, but this probably only becomes obvious in certain circumstances - I KNOW my FAS needs 4 PIPS to sys to be consistently successful as an interdictor, from long experience of doing it. I think there's probably some extra involved, such as the total power in my ship isn't up to running the interdictor properly unless it's prioritised or something, but there is a really obvious drop off in success if I try it with 3 or 2 pips.
A factor that can have a big effect - your graphics card/driver. AMD owners spent a LONG time complaining about the interdiction routine, which we rarely won, until AMD released a beta driver set a few months back that sudfdenly cured all ills - so one tip would be toi make sure your drivers are up to date. (I can only presume that the driver issue caused some sort of lag, and what was on screen wasn't reflecting what the server saw).
Allegedly if your ship is more massive than the interdictor, it's harder to evade - from the interdictor point of view I'd agree that it's a bit easier to interdict a less agile target, but not hugely so.
Sometimes the interdiction ignores everything and flips a coin to decide the outcome.
Overall, it is not a transparent mechanism, nor does it follow clearly discernible rules - the best way to escape is to drop from supercrusie, boost, and supercruise (low wake) or jump (high wake) to another system if the interdictor is mass locking you.
Dave