JSLint will throw the "Combine this with the previous 'var' statement" error when it encounters multiple variable statements within a function. Here's an example in which we attempt to declare two variables x and y with two separate var statements:
In the above example, the variable y is declared regardless of whether the if statement body is entered or not. It is only assigned a value when the if statement body is executed, but it's declared (and will have a value of undefined) no matter what.
The fact that JSLint does not allow you to simply have multiple variable statements outside of the block is just the coding convention preferred by the author, Douglas Crockford. The use of the comma to group variable declarations into a single statement can make the code easier to follow. The use of the comma character in variable statements is documented as follows (ES5 §12.2):
var VariableDeclarationList ;
VariableDeclarationList , VariableDeclaration
It's worth noting that in this case the comma is not interpreted as a grouping operator, but is rather part of the syntax allowed in a variable statement.