You never have 100% bandwidth utilization. You are not the only one using the server you're d/l from. You are not the only one using the network your ISP runs out to your neighborhood. You are not the only one using the network your ISP connects to to access the Universe.
And even if you were, TCP is a connection-oriented reliable protocol, which means it requires acknowledgements of packets recevied, which uses some the bandwidth you want to use for your download. Any erroneous or un-acknowledged packets have to be retransmitted. The sender must get an ack for each packet within a certain time, or he re-sends.
This is opposed to a connection-less protocol like UDP, which basically sprays its data out there, and it's up to the receiving end to make sure it all got there, and to ask for any missing parts.
There is also the matter of re-assembling the packets at the receiving end. They didn't all necessarily take the same path along the interweb to get to you, nor did they arrive in the correct order. Take a sheet of paper, cut it into pieces, number the pieces, put them randomly into ten or 15 paper cups, take them to your friend (except leave three or four out) and have him list each part he got and let you know he got it, and any that he doesn't let you know about, you make again and send.
(The UDP version of that would be for you to send the pieces, and go do something else. When he discovers the missing ones, he should ask for them.)
Also, what else is your computer doing during the download. "Nothing" is an incorrect answer. YOU may be doing nothing but waiting, but your computer is pretty busy. Are you watching a video or playing music during the download? Maybe the disk channel got backed up for a millisecond or 2, which meant the download had to stop because a buffer filled up. Is the buffer the right size anyway? How long did it take the server to respond to the "stop, wait a sec" command, and then to the "OK go ahead" that came after?
