taxable income has nothing to do with the computation of taxable benefits. as a single individual you could have adjusted gross income of $33K before taxable benefits (taxable income would be $21K before benefits)
say gross benefits are $4K 1/2 = $2K
so income for taxability is $33K + 2K = $35K
$25K is subtracted from this (being single) leaving $10K since this is greater than $0 some of your benefits are taxable
if your married instead of subtracting $25K subtract $32K but that would still leave $3K meaning some benefits are taxable.
I think your confusing taxable income with adjusted gross income.
