Recommendations on how to sit properly are not compulsory. Sometimes, it is acceptable to deviate with outstretched or cramped positions to relieve muscle tension.
A "good" sitting position at work can be achieved only by focusing efforts on the three areas that are identified as potentially hazardous:
workplace design (including tasks, workstation, and chair design)
job design
training
None of these areas is more important than the other, and none of them alone can bring about substantial improvement.