Your ISP is allowing your speed to be: 38Megabits/8bits (= 4.75 MegaBytes/sec ~).
The steam server is allowing your speed to be: 2.5 MegaBytes/sec.
4.75MegaBytes is greater than 2.5MegaBytes.
You should be able to download at 4.75MegaBytes or so, because of your ISP.
However, STEAM is "forcing" you to download at 2.5MegaBytes.