The most likely explanation is any non-taxable part of your Social Security. If line 6a of your Form 1040 or 1040-SR is significantly higher than line 6b, this is probably where the additional MAGI amount is from.
Other possible culprits are tax-exempt interest from Form 1040, 1040-SR, or 1040-NR, line 2a, or foreign earned income and foreign housing deduction from Form 2555.
If none of these appear to be the cause, please post back with any other information you might think is relevant.
See Worksheet 1-1, Taxpayer's Modified AGI—Line 2a, in the Instructions for Form 8962 for more MAGI details.
