If you have $55,000 in total other income and are filing as Married Filing Jointly (MFJ), you will not pay any tax on $1000 of long term capital gains.
$55,000 minus $25,100 standard deduction = ~$30,000 taxable income. $81,050 - $30,000 = ~$51,000.
The first ~$51,000 of capital gains will be taxed at 0%.
The "more of your social security is taxable" explanation is the most likely reason for "taxcaster" calculating more tax. See the work sheet on page 15 at https://www.irs.gov/pub/irs-pdf/p915.pdf
