Struggling to understand wire gauge and strands
The stranding is more-or-less arbitrary, determined by the sizes of individual conductor that the manufacturer can make and how they are combined. The differences are (almost) entirely physical, electrically there is very little difference between a wire with many strands (all touching each other) and one with a single core. 

Mechanical characteristics include: 


flexibility
lifetime when bent 
break strength 
corrosion resistance (as mentioned by @Henry Crun) 


The first two generally increase with the number of strands (but the insulation also can have a large effect). 

Diameter variations with stranding will affect the electrical characteristics (eg. inductance, capacitance) only slightly, and rounding from the nominal gauges of strands leads to some slight difference in nominal resistivity though the target is the same resistance per unit length (because the total cross sectional area of the conductors should be the same regardless of stranding). 

Eg. an AWG30 wire has a nominal cross-sectional area of 0.0509mm^2. If there are more than one strand then they should add up to that area. 

For example, here is an excerpt from the standard stranding chart of a specialty wire manufacturer:



They offer as many as 40 strands of AWG46 wire (as standard). The nominal OD increases for coarse stranding in particular, but in this case only by about 20% for the coarsest stranding. 

This particular maker can make single core wire as fine as AWG 56 so you might imagine that (for enough money, and it would be a large amount) they could make an AWG 30 conductor with ~420 strands of AWG 56 wire. 

Note that this only covers the conductor. The insulation system is another subject entirely and pretty much independent of the conductor.