Why does a bunk only take up to a 20cm mattress
Most bunk beds only take a 15cm mattress. Ours can take a thicker 20cm size, the reason why we advise a 20cm mattress is to ensure the guard rails are effective for safety. The thicker the mattress the less effective they will be, so a 25cm mattress as an example will fit but will reduce safety.
The mattress will still be lower that the guard rails but not the required, recommended depth.
If the user is responsible it should not pose an issue but the responsiblity is yours