how come the total capital losses calculated in turbotax are lower than my long term realized losses on my 1099-b?

Where exactly do you see "total capital losses" in TurboTax? What are you looking at?

"Total capital losses" would include other things besides long-term losses. Do you have any long-term or short-term capital gains? Do you have any capital gain distributions in box 2a on a 1099-DIV? Any of those would offset some of the long-term losses.

You should look at the actual tax forms, not a summary screen in TurboTax. Look at Schedule D to see a summary of all of your capital gains and losses.