Why do symmetric ketones form a single oxime and asymmetric aldehydes and ketones can form two isomeric oximes?
An oxime is formed by the reaction of hydroxylamine (NH2OH) with either an aldehyde or ketone. The oxime nitrogen has a double bond (sigma and pi) to the oxime carbon, a lone pair and a single (sigma) bond to the hydroxyl oxygen. If we use valence shell electron pair repulsion (VSEPR) theory to deal with the problem.
Then the oxime nitrogen will be trigonal planar.
If the two groups on a ketone are identical then the molecule will be the same which ever way around we put the lone pair and the hydroxyl. But if the two carbon groups are different then we can have two isomers.
Consider for a moment the 2-hydroxyoximes used as LIX extraction agents for copper. There is a paper from the 1970s on the separation of the isomers of LIX 63 (T.T.Tammi, Hydrometallurgy, 1977, Volume 2, Issue 4 (October), Pages 371-380) which you migh want to read.
To make it more clear we have diagrams here of the two isomers of LIX63.