What I found out is that mil is the exact measurement of .001 of an inch. This is a standard measurement when measuring with a micrometer to determine thickness. Gauge is what we usually measure the steel pool panels by. Gauge does not have a predetermined set thickness. Gauge means approximately close to but not exact. The steel process of manufacture does not allow a consistent thickness, but close to. Only iron metal products are measured in gauge. Metals such as aluminum and copper are measured by ounce. As you go up in gauge the thickness is less. So a 20 gauge panel is about as thick as a sink and a 14 gauge panel will hold up to static pressure such as in a pool better.
So lately many will advertise the upgrade of going to a 27 mil liner for the same price as a 20 mil. Certain manufacturers are able to get a gauge liner and are not held to the exact measurement. The savings for them can be made up over a whole roll of material. However once stretched into a pool this would mean less than usual thickness and shorter lifespan but advertising allows 27 gauges at the same price. Look at the table on the table below where actual measurements were made with a caliper and you will see it does make a difference when looking at the numbers. This is why an experienced person can tell there is a difference between 20mil and 30 mil but has a hard time with telling from touch a 20 mil and a 27 gauge.