Why are some capacitors bent on old boards?
More stress is placed on the leads if there is no space between the part and the PCB. Electrolytics are more fragile than some parts because of the semi-liquid electrolyte, which can leak if there is tension on the lead, such as that caused by thermal cycling.
On some equipment, soft standoffs are used to lessen the strain, and prevent vibration. However, some manufacturers just leave a gap between part and PCB. Even if the part were vertical to start, vibration will knock it off axis. Also, as you state, hand assembly leads to random variation.