I am not sure about tickets ex DXB but if what you are describing is true, it would be consistent with the practice across a number of airlines. There are 2 things usually going on:

1. Customers are often more loyal and hence assign a higher value to flying with their flag carrier (e.g. EK ex DXB). This is due to a number of reasons such as higher identification with the culture of their carrier (e.g. speaking Arabic, carrying local newspapers, staff understand their needs etc), preference fo rthe loyalty programme of the home carrier since they have more opportunity for flying the carrier etc. They are hence willing to pay more for flying them.

2. Airlines with strong hubs can get away with charging higher fares for customers who are O&D at the hub than those that they have to fly in from elsewhere. The reason is that passengers who fly in from elsewhere have more choice. For instance, an oft quoted example in the newspapers is that FRA-LHR-JFK tickets tend(ed) to be cheaper than pure LHR-JFK tickets for same flights and class of service. This is because a passenger originating in FRA could always fly directly to New York on LH, SQ or a US carrier. The rationale is that carriers have a captive audience in their own back yards and therefore can get away with charging higher prices. AFAIK this practice has now been declared illegal in the EU so it doesn't happen as much though airlines still find ways around it (e.g. attaching more restrictive terms and conditions for tickets originating outside of their home countries but for much lower charges)

There may be something else at play here as well:
3. The DXB resident may be less flexible about flying to London than the London resident is about flying to DXB. Also,
4. The LON-DXB passenger expects to pay less for the flight than a passenger who continues the journey on to MLE/ BOM/ CMB etc etc. But the DXB-LON passenger would have to fly out to one of those other destinations first in order to benefit from low prices.

Bottom line, the considerations that affect pricing are numerous and complex. The yield management programmes that look after the decisions of how many passengers of each type (LON based, DXB based, LON based continuing to another city, last minute, advance purchase etc) to allow onto any given flight are similarly complex for the same reason.

Then again, you may have just been unlucky with the dates you looked at.