New plants have a small root zone
A recently planted plant may not yet reach the moisture held deeper in the bed. Water around the root ball and keep checking as it establishes.
Rain does not always land where the roots need it
Leaves, canopies, walls, and sloping soil can redirect rain. Check the actual soil around the new plant rather than relying only on the total rainfall number.
Ease off as the roots settle
The goal is to help the plant establish, not to keep the surface permanently wet. Increase the gap gradually as new growth and deeper roots appear.