Solution
The discrepancy may arise from the definitions of "return" and "re-mating":
Return An unsuccessful mating during the period—a mating that did not result in a litter (the sow may have returned to estrus or may have been culled).
Re-mating A sow that is mated again following an initial unsuccessful mating.
In report 2.5.1 (Boar Mating Results), you can view the number of matings during the period, the number of unsuccessful matings (no litter), and the percentage of returns; these figures can also be retrieved via report 2.2.2 (Mating Key Figures).
Report 2.2.3 (Re-mating Analysis) displays the number of re-matings during the period multiplied by 100 and divided by the total number of matings in the selected period.