spriger chrono test

i know from chronoing this BSA rifle many times over the past 10 years that its a 12fpe gun with the 8.2gr jsb pellets i normally use in it out these pellets so used some BSA elete 7,87gr pellets instead,not surprisingly it came in at 11.18fpe with the lighter pellets which is fine,out of curriosity i put two different 10.34 gr pellets thru it over chrono expecting these heavier pellets to be over 12fpe,not the case,10 shot strings with 10.34 JSBs and 10,34gr H/Ns proved to be a bit of a surprise,both pellets very accurate in this rifle but the H/Ns are avery tight fit in breech and obviously the barrel allso,was expecting approx 700fps from the JSBs and little less from the H/Ns due to their tight fit etc but not to be,averaged 681.36 fps for 10.36fpe,lower than expected,the H/Ns proved evan worse at only 621.64 for only8,87fpe,allmost 2fpe less than the JSBs,being a pcp guy i would normally expect the velocitys with heavier pellets to be less but fpe to increase,but thats not the case with the springer,not really being a spring shooter,is this normal with springers,not a total loss as accuracy remained just fine,but unexpected by me,if anyone familliar with springers can illuminate me on why things work out this way ,i would appreciate any input.