Piwik campaign visits results are 27% lower than in GA !?


Im testing piwik (still on 2.16.1) for school project, but the results for campaigns that piwik shows are way off from Google Analytics results. Overall visits for whole web are similar but campaign visits are in piwik 27% lower than in Google Analytics. Also it seems that piwik does not always record all conversions (goal).

Website visits for October:
GA:	12 258
Piwik:	12 928

Campaign A results:
GA:	921 visits & 19 goals
Piwik:	676 visits & 16 goals 
(Piwik visits records are 26.84% lower)

Campaign B results:
GA:	636 visits & 29 goals
Piwik: 	462 visits & 28 goals
(Piwik visits records are 27.36% lower)

Campaign C results:
GA:	82 visits & 8 goals
Piwik:	68 visits & 6 goals 
(Piwik visits records are 17.07% lower)

I understand that there may be some differences in results but one quarter of (hundreds of) campaign visits seems to me a little bit too much!? I also noticed that for some unknown reasons piwik sometimes didnt record goals that should be recorded. But overall website visits seems to be ok.

Im testing piwik on website with around 3000 actions a day (according to piwik). Piwik and mysql runs on one dedicated server (there was unfortunately one cca 6 hour downtime in October).
Server specs:
CPU: Intel P4 3.00GHz
RAM: 2GB (DDR 400 MHz)
HDD: 1GB - 5400rpm (WD10EACS)

Any ideas whats wrong? Or does this seems to you normal that Campaign visits/conversion results are so much lower on piwik?

EDIT: After comparing “campaign C” piwik visits againts Apache Access Log with “Campaign C” utm_campaign there was realy only 68 unique IPs … is it possible that server HW specs are too low?