Why has my maple got two different kinds of leaves?
If the growth is coming from the base of the plant then this is a sucker. It is simply a branch that has grown from the rootstock onto which the variety has been grafted. Always cut off any sucker growth from below the graft. This is only ever a problem on very young plants, and Japanese maples rarely suffer from this.
Often variegated varieties and varieties of Japanese Maples with extreme leaf shapes only produce their true form on second year or older wood. Young plants of these types will often have two different leaf forms, one juvenile, one adult. Other varieties of Japanese maple may take a year or two before they show off their full adult foliage. Reversion is rarely a problem.