Issue of query of assessment order u/s 143(3)

In one of the case , AO gave the assessment order u/s 143(3) and clearly mentioned no variations proposed.
He gave another computation sheet with addition of some amount in capital gains income ,making the tax payable.
and then subsequently passed the demand notice u/s 156 .

is this possible?
Mentioning no variations proposed in order and charging tax in demand notice is justified?