I think I found the source of the difference:

The $1395 ticket rules show

City Pairs Class Fare Basis Code
LAX - TPE W WLX0ZUMG
TPE - LAX W WLW0ZUMG

The $7 more expensive ticket show

City Pairs Class Fare Basis Code
LAX - TPE W WLX0ZUMG
TPE - LAX W WLW0ZGM3

So the return trip shows a slightly different fare basis code for $7 - presumably a slightly more relaxed set of rules. Then again, the point is probably moot, because the rule also states The most restrictive set of fare rules applies to the entire itinerary.

So my conclusion is that this isn't a glitch, United's computer was trying to sell something that it thought was useful, although with the rule that stated that the more restrictive set of fare rules apply to the same itinerary, the $7 didn't actually buy anything.

Thanks. Found the fare rules. Although my reading of the fare rules seems to suggest that neither are refundable.

WLW0ZGM3
FARE RULES TEXT
CANCELLATIONS
ANY TIME
TICKET IS NON-REFUNDABLE.
WAIVED FOR SCHEDULE CHANGE.

WLW0ZUMG
FARE RULES TEXT
CANCELLATIONS
BEFORE DEPARTURE
CHARGE USD 300.00.
WAIVED FOR SCHEDULE CHANGE.

The difference seems to suggest that for WLW0ZUMG (assuming the entire itinerary was in that fare basis), UA would charge $300 for cancellation and apply the remainder value to a future flight. For WLW0ZGM3 (assuming the entire itinerary was in that fare basis), UA would apply the entire value to a future flight. Does this seem correct to you?

In either case, the point seems to be moot, because the language suggests that if the fare basis is different for the round trip, the more restrictive one takes precedence, so UA would still charge the $300 for the ticket if cancellation is desired.