Systematic component is a portion of offset that comes from assymmetry in design and/or layout (in most cases, anyway). If one takes a sample of 1000 op-amps and calculates the average offset it'll be systematic.
Ideally, good design has zero systematic offset.
Total offset has a random component on top of the systematic one. Usually it's caused by random variations of device parameters from device to device. More exotic reasons include metal and via resistance variations, random mechanical or thermal stresses from packaging, assemby or bonding, and about 1000 other reasons.