Times are a nicety which sometimes fit with the sequence, but unfortunately often don't. Matt Taylor hits the nail on the head when he mentions the numerical order of odd / even numbers. Frequent services tend to see more ordered sequences of headcodes numbered in this way. Sometimes they will skip numbers where a path exists but no service is booked to run - eg. all headcodes may be used in peak times, but off-peak, every other odd/even number might be used for the basic service. For example, perhaps the infrastructure supports half-hourly trains but an hourly service runs off-peak.
As I've posted before, some headcodes are also geographical - you get regional markers, such as certain trains being rendered as 1Sxx if they go across the border from England to Scotland. 1Axx tends to be the primary service group, if there is one, at any given TOC - GTR's Brighton Mainline services, for example. Others take the letter in their headcode from the first distinctive point at which they diverge from other routes. Again, with GTR - and the old Southern franchise before them - 1Exx refers to services to Uckfield, which diverge from the East Grinstead route at Hurst Green Jn, and the first stop south of there is Edenbridge Town, giving the letter E. The first unique station towards East Grinstead is Lingfield - and guess what, most East Grinstead trains are 2Lxx!
