How did hypophosphoric acid get its name?
You are most of the way to the reason yourself:

$$\begin{array}{|c|c|c|}\hline
\text{name} &amp; \text{formula} &amp; \text{oxidation number of P}\\ \hline
\text{phosphoric} &amp; \ce{H3PO4} &amp; +5\\
\text{phosphorous} &amp; \ce{H3PO3} &amp; +3\\
\text{hypophosphorous} &amp; \ce{H3PO2} &amp; +1\\ \hline
\end{array}$$

The "hypo-" prefix means "beneath" or "less than". Hypophosphoric acid, $\ce{H4P2O6}$, has oxidation number of phosphorous as +4, which is in between phosphoric acid and phosphorous acid. Thus, "hypophosphoric" refers to "beneath" phosphoric (but above phosphorous) acid.