How Required Hours are calculated for resource on Horizontal plan in EDD Constrained plan

We have EDD Constrained plan.

When check horizontal plan for some resource, it displays on first bucket huge amount of Required Hours in first bucket (daily).

Every next buckets have adequate amount of Required Hours. It happens every day, after every plan running.

By using statement from the Note: Horizontal Plan Bucket Load Does Not Match Resource Requirement Numbers For Same Bucket (Doc ID 1270113.1)
We found that there are some firmed and some not-firmed work orders having start date in the past and today, plus some planned orders having start date today.

But there are not all work orders using this resource and having past start date.