Actually it does make sense that most heroes start with the same attack speed. The differences may only be due to the fact that the BAT of AM is 1.4 and that of TB is 1.5 instead of 1.7 like most other heroes.
For more information about BAT, look at the dota-allstars forum in the list and guides section...
If assuming the starting agility does not contribute to IAS,
Bone:
+3 agi per level
If he just levels and does not buy any agi/IAS items or put points into stats, his attack speed at each level will be:
Lvl 1: 1/1.7 attacks per second
Lvl 2: 1.03/1.7 attacks per second
Lvl 3: 1.06/1.7 attacks per second
Centaur:
+2 agi per level
If he just levels and does not buy any agi/IAS items or put points into stats, his attack speed at each level will be:
Lvl 1: 1/1.7 attacks per second
Lvl 2: 1.02/1.7 attacks per second
Lvl 3: 1.04/1.7 attacks per second
AM:
+2.8 agi per level
Lvl 1: 1/1.4 attacks per second
Lvl 2: 1.028/1.4 attacks per second
Lvl 3: 1.056/1.4 attacks per second
Buy gloves of haste (+15% IAS)
Lvl 3: 1.206/1.4 attacks per second
Lvl 4: 1.234/1.4 attacks per second
Put point into +2 stats (+2 agi)
Lvl 4: 1.254/1.4 attacks per second
etc
, but i still got some question here.