Attack speed is number of attacks per second.It is calculated like this: 50 / (200 - aspd),
in your case 50 / ( 200 - 173 ) = 50 / 27 ~ 1.85, it means you attack 1.85 times per second.
You increased it 4% means 1.85 + 0.04*1.85 = .1.85 + 0.074 = 1.924, calculated back 200 - ( 50 / 1.924) =
200 - 25,98 = 174,02.
Considering you gained no aspd means you had some decimal in your 173 aspd mean you aspd is 173.xy or 172.xy
im not sure if they round it where xy is decimal part that is not included in equip window.
Edited by vilinkonjic, 04 January 2017 - 11:47 AM.