I don't know why it is, but I did have this experience with the pregnancy I had between DD and DS.

I was a day late for AF and I did a test - it was negative in the timeframe and I threw it in the wastebasket in the bathroom.

About 2 hours later I was in there and for some reason I pulled it out and looked at it again - it had a line.

Two more days of frustration ensued where I kept getting these lines showing up after 20 mins, 1/2 hour etc - never in the timeframe. Still no period either (and I'm only ever late when I'm pregnant).

Went to the doc for a blood test which came back that I was pregnant - but only just. HCG level was very low (hence the faint tests). Finally after about a week the tests began to darken up and I started spotting. A week after that I was still spotting (would have been 7wks at that point) so I demanded a scan to find out what the hell was going on and found out I had a blighted ovum. So that explained why my HCG was so low, why the preg tests were so faint etc.

Anyway the point of that long ramble was to say that they're not always evap lines, so you need to play it by ear. Good luck!