Having some difficulty with TTFB

Hello,

I have some difficulty sorting out TTFB metrics. According to Webpagetest’s glossary, First Byte Time is defined as “The target time is the time needed for the DNS, socket and SSL negotiations + 100ms. A single letter grade will be deducted for every 100ms beyond the target.”

In the tests though what is included in the TTFB result, includes not only the above but also the entire process of the initial DB queries, html output, gzip compression and sending all that to the visitor.
In the Waterfall View, I see the initial dns/ssl negotiaions PLUS the compressed html reaching the visitor measured as TTFB.

Is there something I don’t understand here?

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