I was looking at "asura" article on wikipedia, and found out that certain vedic deities are called asuras. But in Yajurveda Vishnu is equated to Varuna. At same time in Rigveda Varuna is refered to as asura. ALso, in Rigveda, in one place Rama is mentioned with asuratvam (force of asura). Thus both Vishnu and Rama are called asuras. Any ideas why is that all so?