Not a surprise really. Reaching round 3 at Birmingham means usually having to beat a seed top 70 ish, and possibly as high as Sharapova, and you net 30 points. By comparison winning 2 matches at Nottingham will mean probably beating a seed ranked 50-100 (they may even end up being seeded themselves) and netting 40 points.

On top of that it allows them to stay in Nottingham for 2 weeks.