Well according to them when we appear for upfront medical a random IME number is generated so, it takes time for IRCC to link the medical to your application, while when they send re-medical request, they send you IME document which you need to take with you to the clinic and clinic sends the report with the IME number from the document and it is linked to your application easily.
In first scenario, if you get remedical request meanwhile your upfront medical has not been linked yet then it's quite a stressful task to get it linked by raising multiple web forms, also clinics are denying to help you out in such cases.