Sometimes Travel Agents mess about with the Minimum Connection Times as well. Normally the system would consider anything under 2 hours for terminal to terminal transfers as un-achievable at Heathrow.
AFAIK 1 hr 30 is the MCT for T5-T5 transfers with BA.
If you book directly,
normally, the BA system will not allow MCT's below this, there are exceptions to this rule though.
What some travel agents do, upon the wishes of their customers, is book two separate flights within the system with smaller MCT's, as the booking system doesn't see them initially as connecting flights, and then combine the two flights into a single, connecting flight. This effectively circumvents the MCT system, gives the customer 'initially' what they want and then leaves the customer running through terminals, security and immigration trying to chase a tight connection.
As for re-scheduling 10 minutes, it often happens as flight data is updated and route times are re-calculated based upon historical averages.