I see a couple things here, so I'm'a drag them together..
This is because limonite only shows up at the top layers (chert, shale, basalt). All of the sedimentary-only ores give much lower numbers and descriptors than you would normally expect, because the reading for them in 90% of the rock column is zero! Also applies to coal, borax, lapis.. a couple other things maybe?
If you go back to your mined-out ore disc, dig a lil' tunnel out maybe half a dozen blocks in a cardinal direction, and from there dig out some further one-block-high holes four blocks further and nodesearch at the ends of those, you will almost always find more. Ore gets placed in *fields* of discs, all at the same level. Sometimes they get shoved up or down by a block or two because the surface topography shoves the rock column up and down a little, but no further. This even works with iron; I once found three hematite discs in a row, right next to each other, and if there were anything I wanted to do with five or six hundred iron ingots I'd be in business.