Why BMI 21 and not 24.5?

In early 2008 my BMI was 28.5 (182 pd). My November 2008 goal was BMI 24.5 (156pd).  Today at 25.6 BMI (162 pd). My height: is 5'7".What is the evidence that I should try to maintain a lower BMI; 21-24 BMI for life?

BMI 21 - 134 pd (9st 8pd)
BMI 22 - 140.5pd (10st)
BMI 23 - 147 pd (10st 7pd)
BMI 24 - 154 pd (11st)

Source: Obesity: Science into Practice, 2009, Gareth Williams and Gema Fruhbeck
Willett et al
pg 217:
Baseline BM1 22 (x1)
Risk of Type 2 diabetes increased , BMI 23 (x1.5), BMI 24 (x2 relative risk)
Hypertension & Coronary Heart Disease: BMI 23 x 1.25, BMI 24 x1.5
Gallstone disease: BMI 24 (x1.25)
source: Willett WC et al (1999), Guidelines for healthy weight, The New England Journal of Medicine, 341, 427-34

pg 221, fig 9.6, Stevens et al 1998
Cardiovascular Disease (CVD) relative risk 19-21.9 = x1, 22-24.9 x1.3, 25-26.9 x 1.5. Central & visceral adiposity are more powerful predictors of CVD.

pg 226, fig 9.9, Chan et al 1994
Diabetes x2 relative risk >23.9 BMI

New Crabsallover Weight Target

Among male participants who had never smoked, Pischon found the risk of death was lowest at:

1. BMI of 24.5 (crabsallover: 156 pounds (11st 2 pounds))
   
2. 85cm (33.4") waist circumference
3. 0.88 waist to hip ratio (equiv. 96cm hip (37.8") circum.)
   

These are my new goals (see November 2008 Health Strategy - right column).

New weight loss per week target: 0.7 pounds (was 0.8 pounds). 40 week diet completed target: 4th May 2009 to 156 pounds.