That means that only 1 of those items have traded in the time period. It reports the sole trade as the maximum price traded. The -1 for minimum is just to show that there was no second trade at all.
• Scott Gee
Looking at the various time periods, 7-days, 1-month, 3-months, and 6-months, they all show MIN -1 for the Santa Hat. So you are saying that at some point in the last 6-months there was only a single trade on the G.E. for a Santa Hat. It's the same with Black Santa Hat. Could these be items Jagex has taken from a banned account and has used the G.E. to put them back into the game for maximum gp?