QT doesn't! reflect the absolute refractory period, absolute refractory period is a part of the QT but it's not all of it! QT is measured from Q till the end of T wave, so it covers both depolarization and repolarization.
Electrolyte disturbances usually hypo (hypocalcemia, hypomagnesima, hypokalemia) cause enlongation of the QT cause they interfere with the normal repolarization process, if you asked about hypocalcemia then practically in diastole there is a channel which is working Na/Ca exchanger if you keep the calcium levels outside low then it may increase the activity of the channel, increasing by somewhat the Na entery to the cell as you know this antagonizes the normal repolarization process (again this is under theory). Antimuscarinic drugs play with the QRS, they change the K currents, you can see that other drugs like class III anti arrhythmics with also block K+ channels have the same effect. I dont think that you need to drill in this topic too much just know which drugs cause it, and what can be the consequence.