XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (362 page)

So, these examples show some of the surprises in store:

• You can't assume that
  $X=$X
  is true. It usually will be, but if
  $X
  is an empty sequence, it will be false, because there is no item in the first sequence that is equal to an item from the second.
  
• You can't assume that
  $X!=3
  means the same as
  not($X=3)
  . When
  $X
  is a sequence, the first expression is true if any item in the sequence is not equal to 3, while the second is true if no item in the sequence is equal to 3. Generally speaking, it is best to steer clear of the
  !=
  operator unless you know exactly what you are doing. Use
  not(x=y)
  instead; it is more likely to match the intuitive meaning.