Drift on WSPR

When I`m active on 10mb or 15mb I can note more drift on WSPR transmissions by many stations, like this:

Timestamp Call MHz SNR Drift Grid Pwr Reporter RGrid km az
2009-04-22 20:36 W5CGC 28.126048 -15 0 EM13ka 10 WD8INF EM79vk 1359 55
2009-04-22 20:36 CX2ABP 28.126137 -10 -1 GF15wc 5 K4XL FM07ln 8397 341
2009-04-22 20:34 WB5NMZ 28.126080 +10 4 EM62vi 5 KU4PY EM62wj 9 59
2009-04-22 20:34 WB5NMZ 28.126092 -1 2 EM62vi 5 N4AU EM62vp 32 0
2009-04-22 20:32 K4XL 28.126113 -25 1 FM07ln 10 PY2RML GG67 7473 148
2009-04-22 20:32 N4AU 28.126099 -20 3 EM62vp 1 KU4PY EM62wj 29 164
2009-04-22 20:32 WB5NMZ 28.126071 +7 4 EM62vi 5 KU4PY EM62wj 9 59

What's the reason of this? Slow response of receivers? Transmitters not accurate? In 40mb I canĀ“t note any high drift values.

I have test my own xmsns on 10mb with 2 rigs: Kenwood TS-50, 10w and Realistic HTX-100, 5w.

Any ideas?

73 & DX de Rod, CX2ABP.