Tracking outbound links for CPC measurement

I have a website – https://www.northernlightsiceland.com – and I need to measure how many times a visitor clicks on a ad on the sidebar and intext links etc – a typical CPC setup. I set up Google Analytics event tracking and though it was working fine – I got some measurements etc. But when I asked my client how many time he saw as Referrals from my site then he gets like 33% more clicks than what my event tracking shows on my site. This is causing problems because I charge based on number of clicks? Anyone know why this could be? Or do anyone know a better system or ways how to do this?