Just Wondering where this is in the entire scheme of things....
I'm only new; a few months in, but layer numbering has this variation.
My job slices in Cura shows layers 1 to 170.
Octoprint, OctoDash & Print Monitor tell me I'm first up printing layer Zero.
The add-in Dashboard Says Layer 1.
(Which matches the first layer in Cura).
I know it's minor, but is there a need for this inconsistence?
I do have a Mathematical background so can understand why the first layer may be refered to as Zero, but it's just he slight mindset that's required to +/- between the two values that needs a rethink on occasions.
There will be different implementations of counting in the different places you are looking, by different, and they've decided that's how they will count.
The only thing that counts layers in OctoPrint is the gcode viewer, and it's often likely this is different from your slicer since it operates from Z changes - start gcode including prime lines can easily throw this out for example.
The rest are 3rd party implementations - if you have DisplayLayerProgress, that does it one way (and I think this is what OctoDash uses). Not sure about 'Print Monitor'. The Dashboard plugin does calculate it's own way as well.
I don't think you'll get an answer as to why they're different without asking the plugin authors directly.