Interesting information about underground power lines

There's been a lot of talk in recent weeks, after the large storms that disrupted power in several mid-Atlantic states, about the prospect of burying power lines. Overall, according to the Edison Electric Institute, about 18% of distribution line mileage is underground. (That's the small lines that go to your house, not the big transmission lines. Of those, about 0.5% of mileage is underground). The chart above, a screenshot I took from The US Energy Information Administration site, tells why: it costs considerably more to run new distribution lines underground, or to convert overhead lines to underground, than it does to install new overhead lines. As I read the chart, considerably more means four times as much per mile, though according to accompanying article it's "up to five to ten times more..."

What accounts for the difference? It's partly the circumstances - what's under the ground: granite? Water? And there's a cost for dismantling overhead lines during the replacement. But part of the difference is the graininess of the charts - I mentally rounded up the cost of new overhead lines to $500/mile, and rounded down the costs of new underground or conversion costs to $2000. That makes sense based on the charts, given the $500 increments. But if the chart and the text are that different perhaps there's something wrong with one of them. Notice also, about the chart, that the bars do not go all the way down to the x-axis, but float a little above it.