In the past the system for tracking delays and EATs used to be manned by a Heathrow approach valid atco.
Now, as there aren't enough Heathrow valid controllers around to be able to look after it it is generally left to a Supervisor who also has a myriad of other tasks to attend to as well.
As such when delays are building often no-one is actively tracking this and it's not until one of the approach atcos turns round and shouts at someone that they need to devote some time to the EAT system that said supervisor will take a look at it.
From that point it then takes time for the system to be brought up to date and reliable information gained from it.
It's not unusual for the system to be showing inaccurate delay info i.e. saying delay less than 10 minutes when it's actually 15 or more.
I don't think you'll find any of the approach controllers are particularly impressed with this, we don't enjoy being made to look stupid by giving out delay info that is simply wrong because an important tool in the system is not being actively looked after.
In your particular case I expect it was just the volume of traffic that led to your total delay and that the earlier info you were given on 10-15mins was just inaccurate info for the reasons stated above.