The relationship between Hard commit and UoM

Dear Experts,

Please explain me about hard commit function from case below

Case 1 :

1. Create item master with UoM = EA

2. Create SO, linetype W with UoM = EA (100 EA)

3. IC (100 EA)

4. Change item master UoM from EA to BK (standart uom -> 1 BK = 20 EA)

5. Shipment confirmation (20 EA)

6. In item availability, SO/WO Soft Commit -20, but SO Hard Commit +19

I would be very grateful for your help..

Sincerely,

Oviedityanto