There are several possible reasons for that:

1. Processing delay: Budget optimiser experiences a two-day lag in processing data, which can result in a discrepancy with ad platforms, particularly for recent target dates. Conversions may still be attributed during this time, causing the numbers to appear lower.

2. Data fetching criteria: Budget optimiser retrieves data only for days with clicks or Meta landing page views. If your campaign had days with no activity, the data for those days, including conversions, spend, and revenue, will be absent, resulting in lower reported figures.