Difference between manga and light novel written by same author
A manga is a comic book or graphic novel - a story told in sequential art.
A light novel is a prose book with some illustrations. Called "light" because the content tends to be easy-reading and the size is small and portable.

I googled it myself and the first search result was this below:
https://www.quora.com/What-is-the-difference-between-manga-and-light-novels 
And honestly you won't find a better answer to this. Question is pretty much the same as well. You just added the same author clause.

By looking at the timing of manga and light novel(almost the same) i can just say that, either he wasn't ready for someone to work on his creation or he wanted to expand his fanbase while he was onboard to earn money or fame or even just the simple logic "like providing fans his content as per his vision". He, decided to write both novel and manga.