This is an explanation from a medical site (see the link.)
The amount of progesterone in combined continuous HRT is small. It's there to offset the dose of estrogen in the preparation. In that type of HRT the 2 hormones are very carefully balanced to work together to keep the lining thin.
In women who still have periods, they produce their own estrogen. The amount can vary when periods become irregular.
The dose of progestogen in combined continuous HRT is not enough to offset your own estrogen AND estrogen in HRT. The outcome is usually erratic bleeding where your own cycle breaks through (because the HRT progestogen isn't high enough to control it.)
https://www.menopausematters.co.uk/postmeno.php
POST MENOPAUSAL -
Continuous combined therapies.
"Period free" or continuous combined therapy can be used by women who are 54 + yrs, or more than one year since last period at any age.
The criteria should be fulfilled in order to offer such treatment to women who no longer have a continuing ovarian cycle, so that steady levels of both estrogen and progestogen can be achieved.
When there are steady levels of estrogen and progestogen from daily administration of both, the womb lining stays thin.
Although some bleeding in the first 6 months of therapy is common, there should not be bleeding after that and the lining does not go through the stages of stimulation and then shedding as it does during a normal cycle and with sequential therapy. Start with low dose preparations and increase as necessary for symptom control.