Final answer:
The difference in standard deductions between Sam and Abby is because Sam has unearned income and qualifies for a lower standard deduction as a dependent, while Abby has earned income and her standard deduction is the same as a single filer with income less than the deduction amount.
Step-by-step explanation:
When dealing with personal income taxation, the standard deduction is an amount that reduces taxable income. The size of a taxpayer's standard deduction can vary based on their specific financial circumstances. Sam and Abby have notable differences in their standard deductions because Sam appears to have a standard deduction as someone who can be claimed as a dependent by someone else, which is typically a lower amount, while Abby's standard deduction suggests she cannot be claimed as a dependent and is therefore eligible for the same standard deduction as a single filer whose income was less than the standard deduction amount. In general, Sam's lower deduction suggests his $8,490 is likely unearned income (from investments or similar sources) and Abby's higher deduction suggests her $8,490 is earned income from employment or self-employment.