We battled with this with 2 boys.
I discovered some reasons for the lying.
1. too lazy to do the right thing. (so I gave them more activity to help them build up strength and endurance. LOL) Or sometimes I made them 'rest'- no tv, no books, no computer, no music, no games, etc.
2. too absorbed in an activity to move--so they need to learn self control and self guidance skills. So I set timers, checked on progress of assignments (which can be anything from picking up toys to writing spelling words), and made lists for them to check off things.
3. too lazy to care and confront life in general. They just want to play, play, play, play. So we did all the above and sometimes added punishment like spanking, time out, whatever works for that kid.
Occassionally you do have kids that just want to please you and will do anything to keep from having you or someone else get upset. They need to learn to care more about the truth than getting in trouble, seeing you upset, seeing anyone else angry with them, etc.