WAEC and NECO (2023) unveiled 16.29% of WASSCE Mathematics learners’ results were withheld owing to assessment malpractice, while only 84.38% of WASSCE Mathematics learners merited credit, and 61.60% of NECO SSCE Mathematics learners merited credit in the Mathematics assessments oversaw by both assessment bodies. These issues, with others, necessitated this exploration to assess the disparity in item statistics between 2023 WASSCE and NECO SSCE Mathematics large-scale assessments via IRT Three-Parameter Logistic Model. Three postulations were posed. This exploration applied survey research design. While this exploration’s populace entailed 31,182 SS 3 Mathematics learners in 550 public and 850 private senior high schools in Enugu State, a multistage sampling tactic opted for 9,724. 2023 WASSCE and NECO SSCE Mathematics fixed-choice tests were applied as instruments for this exploration. Data amassed were probed via 3PLM within IRT framework, using flexMIRT psychometric software to produce the slopes, thresholds, and lower asymptotes, and independent samples t-test to verify all postulations for significance. The outcomes of this exploration proved a significant disparity ensued in the lower asymptotes amid 2023 WASSCE and NECO SSCE Mathematics fixed-choice items. Nonetheless, no significant disparities emerged in the thresholds and slopes amid these assessments’ items. The realistic connotation of these outcomes inferred that harnessing IRT Three-Parameter Logistic Model could better spot Mathematics fixed-choice items formed by WAEC and NECO that are vulnerable to conjecturing, conceding for their deletion or amendment to mitigate guessing by learners in successive assessments, thereby boosting the entire attribute of their items.