Why aren't BeMob and PopAds metrics aligned?

[AffiliateNate]

Hi,

I am using BeMob as my tracker and used PopAds as my traffic source in a campaign. Any idea / likely guesses why the amount “spent” on PopAds doesn’t match the “cost” on BeMob? (Is this normal? Asking as a beginner)

• August 1: 4.66 spent on PopAds but $2.322 cost on BeMob
• August 2: 4.01 spent on PopAds but $1.790 cost on BeMob
• August 3: 0.72 spent on PopAds but $0.200 cost on BeMob

Thanks!
Nathan