there is a differenze in thinks that technical true and what works good
the dropdown shows option that good not all that can work (related to codec settings) not every aspect ratio works good with encoder related to the compression algorithm. related to the encoder you have a lower range of resolutions that are good
you can select what you will but loose quality on same resolutions by fill bits
means for example that you can select 936P but the encoder encodes a 1008P related to the 4x4/8x8/YxY pixel compression Algorithm (the numbers only phantasm Show Pictures noch checked if the numbers match, its only one reason behind why there not show every technical possible)