Well, my first reaction is: What makes you think that melting
and burning SHOULD be the same kind of property ?
But I'm digging for a way to answer the question the way
you asked it. Here's one way I can think of:
In order to melt, it doesn't need any other particular substance
to be there, and no new substances are formed. It can be in air,
in water, in vacuum, or in jello. All it needs in order to melt is heat,
and when it's done melting, the substance in the puddle is exactly
the same as what was there in the solid block.
In order to ignite, it needs another particular substance to be
present, usually oxygen. When it ignites, it begins to form new
substances, by combining with the other substances around it.
And when the other substance is all used up, the original one
is no longer flammable, even if there's plenty of it left, and
no matter how hot you make it.
When you look at it that way, the two processes are very different.
