Im not an expert in this subject and am going to say what I've heard and read so take with a pinch of salt.

From what I understand bramha is worshipping shiva knowing him in his original form as an expansion of sankarsana. This sankarshan as far as I know is where the 3 vishnus are expanded from, and from maha vishnu the main siva expands from his glance to impregnate material nature (durga devi) then he resides in between the spirtual and material worlds (then a shiva in every universe expands). The orginal shiva is sankarsana (sada shiva???) in vaikuntha who is the cause of the expansions of 3 vishnu and shiva, including krsidaksha vishna in every heart. Therefore in his original form shiva is also paramatma.

Thats the way I've understood it and may need to be corrected.