what observations show the large number of ABZ transfers on IAH-LHR?
First of all let's be accurate about what I said:
from observations on many LHR-IAH-LHR trips there is still a good deal of travel from and to between IAH and ABZ
There's quite a difference in both words and meaning between my sentence and yours. Also I didn't specifically use the word "transfers".
Now to the detail. There is a large British ex-pat community in the greater Houston area which works in the oil and gas industries. Many of the international companies based in Houston still have bases in Aberdeen and many of the families maintain a base in the Scottish city or have family there. From observation at check in, talking to people in the departure lounge and, more substantively, having a wide range of contacts with people employed in many of the major companies, Aberdeen is a significant destination and source of passengers to and from Houston, be they on business or personal travel. There are currently three ways most people make the trip. KLM over AMS, Air France over CDG, both offering connections and through ticketing, or BA and United to (note TO) Heathrow. Whilst business travellers tend to transfer at LHR to an ABZ flight, many personal travellers spend some time elsewhere in the country either visiting family or just vacationing before travelling on to Aberdeen by road or rail. There has been dissatisfaction with the standard of the equipment and service to Heathrow, particularly the state of BA's 747s, but the route now has a 777 on the 194/5 and a 787 on the 196/7.
Those who use KLM and Air France would generally prefer a UK transit and choose AMS or CDG to either avoid LHR or UA/BA or both.
Eventually there will be an upswing in the oil business and not only will Aberdeen be a player but Manchester also has significant oil related equipment manufacturing companies based in its catchment area. Houston is very much the administrative centre for most of the world's oil and gas companies outside of Russia. It has limited European connections for a city of its size, only AMS, CDG, FRA, IST and LHR being served. From a gas and oil perspective, Aberdeen, Glasgow, Inverness, Norwich, Oslo and Stavanger are either major centres for the industries or have the industries in their catchment areas, all being served from MAN, so there is potential for onward connections or code shares.