Its pretty unusual in this game for ships to alter course. It does happen but rarely.
Most likely they spotted you and turned away from you and showed you their stern.
The other possibility is you took a quick nap in time compression and the ship crossed your bow. 12 degrees is well within an acceptable solution angle and once the ship gets within ... well 12 degrees or so it crosses zero (your bow) pretty fast when compared to when its at 30 degrees or so. Exponentially faster even
You can do the math to prove it to yourself simple algebra.
Oh.... Ive done the same thing. Leave it a TC=8 get distracted for a moment and phooey there goes the shot.