Differential pairs must have clearance from adjacent aggressor traces or victim traces, including other differential pairs. Routing differential pairs too close to other pairs decreases noise immunity.
In multilayer PCBs this applies to traces on adjacent layers as well. Orthogonal trace crossings are preferred between traces on adjacent layers not isolated by a plane.
In practice, there are usually many different geometries of trace width and spacing which are "correct". We strive to find a geometry that will fit on grid. The effort pays off in simplicity, faster layout, better autorouting, and easier rework.
