Some ISPs use a "boost" that gives you a surge of bandwidth for the first $n seconds of a transfer. Sometimes it's long enough to sustain an entire SpeedTest/SpeakEasy/[favorite bandwidth test here] test.
Another possibility is that your ISP is shaping traffic based on the application. ISPs have long been known to tamper with torrent streams. Calling up your ISP and trying to get answers will be slightly less pleasant than self inflicted oral surgery using a reciprocation saw. If you get a direct answer it will take weeks of phone calls and abandoned tickets. It will also be wrong. If the answer happens to be right, it will be wrong shortly.
Finally, as David Schwartz mentions, you may be confused on the units of measure at play. Most network traffic is measured in "bits" and not "bytes." An internet connection of 30MB is quite different from one rated at 30Mb. Speed tests almost always rate traffic in bits so SpeedTest is telling you that you are getting 30 mega bits. Perhaps whatever utility you are using to download files is giving you the bandwidth rate in mega bytes.
If you want a more solid connection with no fear of ISP tampering, you'll need to pay for a business line and then inspect the SLAs associated with it. If it's "business class" DSL or Cable, just know that the ISP is laughing at you for paying more money for the same awful carrier signal.
MPLS over an OC12 or go home.
Is there a chance you are getting throttled? – soandos – 2011-12-27T03:15:44.787
3Your units are borked. I assume your speedtest.net numbers are 30Mbps/1Mbps? What are your second set of numbers? 1MB/s 100KB/s? Or something else? – David Schwartz – 2011-12-27T03:37:56.540
A 30Mbps connection should give you a maximum transfer speed of 3.2MB/s. A 1Mbps connection should give you a maximum transfer speed of 110KB/s. (These are computed values. The measured values are about the same.) – David Schwartz – 2011-12-27T05:42:45.873