Why do I keep getting these "rejects"?

"
 [08:32:21] Rejected untracked stratum share from Pool 0
 [08:32:51] Rejected untracked stratum share from Pool 0
 [08:33:17] Accepted 1c029571 Diff 2.34K/512 GPU 0
 [08:33:27] Rejected untracked stratum share from Pool 0
 [08:33:57] Rejected untracked stratum share from Pool 0
 [08:34:14] Accepted 46790c7c Diff 930/512 GPU 0
 [08:34:27] Rejected untracked stratum share from Pool 0
 [08:34:59] Accepted 15ddf446 Diff 3K/512 GPU 0
 [08:35:05] Accepted 61ebd012 Diff 669/512 GPU 0
 [08:36:07] Rejected untracked stratum share from Pool 0
 [08:36:37] Rejected untracked stratum share from Pool 0
"