That's the limit of what your ISP should cap you on. You can never be sure of this, since Cox users get far more than they pay for.
Basically, they're advertised speeds, but most never reach them. It's the same with harddisks, advertising that they can hold 40 GB, while it's about 37-38 in fact.
If you reach 90% of your advertised speeds you're getting what you pay for, since that 10% tends to be lost in overhead. Also remember that the server you're trying to download a couple of bits from might be too far away to guarantee the fast speeds. I lose 200 kbps when downloading from testmy.net and my ping is about 100 - 150.