Why was my 2015 standard deduction and personal exemption $5,091 and $2,100 when they should have been $6,300 and $4,000, respectively?

It's called a phase out due to income limitations ...

• The standard deduction rises to $6,300 for singles and married persons filing separate returns and $12,600 for married couples filing jointly, up from $6,200 and $12,400, respectively, for tax year 2014. The standard deduction for heads of household rises to $9,250, up from $9,100.
• The limitation for itemized deductions to be claimed on tax year 2015 returns of individuals begins with incomes of $258,250 or more ($309,900 for married couples filing jointly).
• The personal exemption for tax year 2015 rises to $4,000, up from the 2014 exemption of $3,950. However, the exemption is subject to a phase-out that begins with adjusted gross incomes of $258,250 ($309,900 for married couples filing jointly). It phases out completely at $380,750 ($432,400 for married couples filing jointly.)