They haven't asked for 'most likely' but 'lowest probable' and there is
a 30% probability of a 1,500ft cloud base. Remember that these questions may
have been written in one European language and translated into another European
language before final translation into English by someone who doesn't speak
English as their first language. An English speaker would probably use the word
'possible' instead of 'probable' in this question and it would have been
clearer. Well, quite possibly they would.
BECMG 1315 changes the lowest cloud base to 1,500ft (1 to 2 oktas Cb) but there is a 30% probability (PROB30 TEMPO) that
there could be 3 to 4 oktas at 1,000ft.