Conversion lag refers to the delay between people clicking an ad and when they purchase a product on the website. This lag can be significant and changes between niches & websites. This lag creates a late conversion which is attributed to the original ad click.
Conversion lag happens because the user's purchase journey is not linear most of the time. Most users purchase journey doesn't look like this:
Seeing an ad -> ad click -> purchase
It looks much more complicated, something like this:
Ad click -> didn't purchase -> 2 days later came from organic search -> didn't purchase -> saw an ad -> came from direct -> added to cart and didn't purchase -> 3 days later saw retargeting ads on high frequency -> came from organic -> purchased.
Therefore, you might see that our results of the last recurring period get better after a few days/weeks - it's because late conversions came because of the last period ad budget.