setting pulse width time value for triggering

Is the time value used for this the value of delta-T as set by altering T1 & T2 on screen? If so, does the position of the lines (for a given delta-T) matter as far as triggering is concerned?
I ask because I had set delta-T to a particular value then used the >TH Trigger condition to capture a waveform. On inspection of the captured waveform though, TH was reported as being less than the value of delta-T.
Thanks for any advice given!