The reassessment period varies from person to person - and even from one decision to another for the same person. Nor does everyone always have to attend a 'medical' with an ATOS HCP. It may be that the Decision Maker feels s/he has enough information this time, without subjecting you to the medical.
I don't know anything about the letter asking you to see someone at the JobCentre if this is extra to the interviews you have already been undertaking.
Nothing on this board constitutes legal advice - always consult a professional about specific problems