How does this work? does it work does -.1 aspd mean -.1 hit/sec ???

How come a sling w/ -.1 aspd becomes on .76 hit/sec instead of .81 hit/sec?? any way to calculate this?

things i've noticed melee weapons w/ -.1 atk interval gets +10% more hit/sec
for example dual hammer which is .83 becomes .91 w/ -.1 atk interval
dual swords which is .91 with -.05 atk interval becomes .95

but it's different with range weapons....
bow = .67 w/ -.1 atk interval becomes only .71
xbow= .62 w/ -.1 atk interval becomes .67
sling= .71 w/ -.05 becomes .74
sling= .71 w/ -.1 becomes .76

really dunno how the calculations go