We own a webshop and are slowly our visitor numbers every month with about 10%, but our New vs. Returning numbers remain at about 30 vs. 70%. How could this be?

I know it's sortof a good thing because as we increase our visitor numbers the same amount of them will keep coming back, and sales have been great, but I find it so strange it stays so steady.

Does anybody else experience the same?
What's a healthy ratio?

That's weird indeed. You might be having a very solid way to make your visitor numbers increase. Try something different and see what happens!