I am able to draw the same thing, using both Polyline and Path, but when it is rendered, I see the difference. That is why the question -
<svg xmlns="http://www.w3.org/2000/svg">
<polyline points="200 100, 220 120, 280 120, 300 100" style="stroke:red; stroke-width:2px; fill:none" />
<path d="M200 100 L220 150 H280 L300 100" style="stroke:blue;stroke-width:2px; fill:none" />
</svg>
You see the horizontal line, although both are 2px but one appears thinner. Why? View it here https://jsfiddle.net/xeafLqjp/