Minor grid lines on xy line graph will not plot properly...specifically y-axis

I've combed through the forum and haven't seen anybody with this problem, but it feels like a rather simple one. (y-axis) I am attempting to plot minor gridlines with a 10:1 ratio to the major gridlines (10' minor/100' major - min/max & all units fixed). It will plot the lines, however they do not "sync" with the major grid lines (100') and plot ~9.5:1. I've never seen this, and it does not make any logical sense.

The only ratio that does seem to work and plots correctly is 20:1 (20' minor/100' major) and that is when I establish the minor grid lines as automatic.

Furthermore the minor grid lines plot over the major grid lines. This looks sloppy. Is there a way to place the minor grids behind the major lines? I've attached screen shots because the data is confidential. Hope someone can help. Many thanks.

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