In an oral culture — one that is not written down — mythology evolves as it is passed from storyteller to storyteller. The Jesus myth was created in exactly this way, pasted together from earlier sources. This process is called ‘syncretisation.’
There is no fixed record of an oral tradition, by definition. In an oral culture or tradition, myths grow and develop to reflect the lived experiences and cultures of the people telling them. It was only when writing was invented that these traditions could be codified and by that time, they had been evolving for thousands of years. This means that there are many versions of the same myth, as different peoples carried it forward.
So we cannot say that, because detail differences exist between two similar myths, they are different or have different origins.