Yes I agree with Birdy & Sleepless that can be an issue, people may log off for any reason.

However, sometimes you recognise same hosts. They have same few shared files ... but with a few extra ones. (Or same ip address or similar & uploading same type of files from me, or some of my files in their shares, etc.) But they seem to remove the extra files when you start to download them.

Slightly different scenario:
A person who has been uploading my files over a few nights, I chose to download one song from them on the first night. It immediately went into 'Busy Host' status. Each night it remained in that status the entire time for 1-3 hours each of those nights. Occasionally I'd try to push it, using the resume button or cancel & reselect from listing (being careful not to push often & get banned for the session). They simply logged out after finishing uploading their files from me. This person was sharing over 300 files. Makes me wonder if all their upload slots were really so busy, or whether they had any upload slots at all. None of the files they downloaded from me were in their shares.

The above scenario is common with hosts who use Shareaza for example (they use multiple networks). However this was a LW host. Perhaps they were uploading a torrent to someone; anyone's guess.