CrUX bug? Since multiple months CrUX displays other data as Lighthouse test

Question

In my understanding, CrUX data is an average of last 28 days, where the Lighthouse test is a live test result in a controlled environment. So how can it be, that CrUX displays since multiple 28-day timeframes other results, as Lighthouse displays in the same long time?

You can see the data discrepancy on the screenshot below. These data, both of CrUX and of Lighthouse, is same in a timeframe of five months: CrUX CLS is red, Lighthouse CLS is green.

How can it be? Is it a CrUX bug, or do I misunderstand/misinterpret displayed results?

Answer 1

Saw this also on the CrUX mailing list so posting same answer I gave there, here.

This suggests one of two things:

1. A different page is causing the CLS than the oost being tested. Note that PSI is only able to provide Origin-level CLS and not specific data for this URL:

or

2. Your shifts are caused post-load, rather than from the load that Lighthouse can check. This could be due to scrolling causing CLS (e.g. lazy loaded images without dimensions) or CLS that happens due to an interaction but after the 500ms grace period. CrUX measures CLS across the whole page life cycle from real users.

Both of these issues are discussed in the Optimize CLS guide: https://web.dev/articles/optimize-cls#understanding_where_your_shifts_are_coming_from