Adwords and Piwik visits doesn't match (no replies)

I'm using PIWIK about 2 months now, and it works just fine, but now I'm stuck with this:

I launched a campaign in adwords for one of my client's website, and I attached campaign and keyword ID tags for the landing URL to be able to collect data in PIWIK about the CPC campaign.

The URLs are like this:

https://www.dijnet.hu/ekonto/control/szolgaltatok?pk_campaign=adwords&pk_kwd=szolgaltatok
https://www.dijnet.hu/ekonto/control/elektronikus_fizetes?pk_campaign=adwords&pk_kwd=elektronikus_fizetes

etc.

My problem is, that my Adwords campaign shows about 20% higher click numbers, than Piwik displays for adwords campaign visits on the site. Unfortunatelly I can't find the source of this. I checked the Entry pages datas in Piwik (the landing pages are couldn't be accessible anywhere else, only from the Adwords campaign) and the visits showed for entry-s are equal with the pk_campaign=adwords datas in Piwik/Referrers/Campaigns.

Any idea what could cause such difference?

Thank You in advance!