sample runs slower than ladder? - (Oct/26/2013 )

my sample always kind of runs a bit slower than the ladder. sample is vector around 7.3 kb which is digested and linearised, so theere might be some supercoiled and some linearised but i always have sample running around 10 kb at 30 minutes, and then at 1 hr the sample comes parallel to the 7.3 mark of ladder. why is it so?? ITS A 1% GEL IN TBE.

Variations are often caused by excess salt in the buffer used for your DNA sample. Longer run times will diffuse the salt away from the DNA sample, and may account for the more accurate response for long run times.

thanks bro..