Of the two backup methods, "full" and "incremental", the latter is in my opinion incorrectly labeled (or maybe incorrectly documented?).
Documentation says: "If the Incremental option is selected here, only those files that have changed or were created since the last full backup will be included."
Usually, for this method, the term "differential" is used. Difference is as follows:
"Incremental": only those files that have changed (or were created) since the last FULL OR INCREMENTAL backup are included.
"Differential": only those files that have changed (or were created) since the last FULL backup (disregarding any intermediate differential or incremental backups) are included.
Going with that terminology, the backup method employed by Virtualmin should be labeled "differential".