20% Less Conversion Tracking With Piwik Than With Google Analytics

Hi folks,

I have a problem with piwik conversion tracking which has been bugging me for quite some time now and after countless research I am running out of ideas where the problem might lie.
The thing is: On our site we use Google Analytics and Piwik for conversion tracking. While both platforms succeed in tracking conversions we usually track 20% less conversions with piwik than with
Google Analytics.

We are running a piwik conversion tracking script which has been constructed on the basis of the details given here Ecommerce Analytics - Analytics Platform - Matomo . It is just a simple javascript, which first parses the conversion data from the page (for simplicities sake from the google-tag-manager data layer) and then sends the ecommerce tracking request as described in the piwik docs.

First I thought the problem might be that the ecommerce-tracking requests sent to piwik are sometimes incomplete (i.e. lack data). However I can rule this out now as I have constructed a side script which logs all request data before it is sent to piwik. There you can see that there is no difference between the amount of information sent to piwik for conversions which are tracked versus those which are not.

Then I thought that the problem might be that Piwik might have problems with German unicode characters (as we are a German shop) but I could not find any significanct pattern there either.

I also do not necessarily think that it has anything to do with https as both piwik and our shop have a SSL-certificate. However when we just started to experiment with piwik we had some problems there as we first
did not have a SSL-certificate for piwik. This has been obviously changed by now but it might be that we have still an old version of the tracking script hidden in the depths of our sever which still might try to send data to piwik (but to the old “http domain” and not the new “https” domain). Do you think that the problem might be there?

Otherwise I do not really have any other clue. And as I do not think that my last idea is a promising hint I wonder where the problem might lie otherwise.

Therefore I please ask for your help. Do you have any idea what might cause the tracking difference of 20% between Google Analytics and Piwik?

I am grateful for any advice! Thanks!