Standard weight is 11.34g. 11.5 is probably well within tolerance. How good is your scale? What weight does it give for the other clad coins in the roll?
Actually it is closer to 11.6. The other coins in the roll weigh 11.3 But there was another 1980 P that weighed 11.1 I weighed a 1965 that I have and it weighed 11.6 So I think my scale is in line.