There are many recommendations about double bonds in the Graphical Representation Standards for Chemical Structure Diagrams (IUPAC Recommendations 2008). In particular, the general rule GR-1.10.1
If a double bond has more substituents on one side than on the other, the double bond should be offset to that side.
explains your second example (N-methylbutan-2-imine). And GR-1.10.2
Double bonds with two or more substituents on one end and no substituents on the other should be drawn with the two segments of the double bond centered relative to its atoms. Double bonds of this type are necessarily acyclic, and are most commonly found in carbonyl and acid functional groups.
explains your first example (butan-2-one).
Reference: Brecher, J. Pure Appl. Chem. 2008, 80, 227-410; doi 10.1351/pac200880020277 (open access).