Theoretically, wouldn't the Adobe single page visits metric match a metric I create where count(distinct page) =1? Both should just be pulling visits where only one page was seen. I also limited it to where only one page and one page type was seen within a visit, but the pre-formulated single page visits metric is still always significantly lower than the metric I created. Would really appreciate any insight into what additional kinds of visits are being eliminated in the Adobe single page visit metric that are causing a mismatch between the two metrics.
If you are using count distinct Page Views, I don't expect this to exclude non-page view hits (custom links). So, I assume the segment is higher as it is including visits that had 1 page view and then 1 or more custom links. The default segment is visits that only have one hit (Page view and/or Custom link).
out of my mind I see one big difference: page reloads. I assume that those visits are not counted to the "single page visit" (since they have more than 1 page view), but they are included in your segment.
another difference might be if you apply another segment to the project. while the default metric is nit affected by any project segments, your custom segment might be...