FlyerTalk Forums - Understanding YQ/YR Calculation

Hello all. I am wondering if anyone can explain if there is a non-"black box" way to interpret how YQ/YR fees are calculated on international flights. I was looking at this Delta itinerary for YUL<->HND:

Code:

YMQ DL X/ATL DL TYO Q10.96 183.44VKE0WNMO DL X/DTT DL YMQ Q10.96 183.44VKE0WNMO NUC 388.80 END ROE 1.368282 XT 19.40CA 26.20SQ 1.31XG 2.61XQ 13.04YC 14.00XY 7.92XA 11.20AY 21.30SW 7.20TK 650.60YR 4.50XF DTW4.50

You can see that the YR fee is $650 here. Out of curiosity, I took a look at another Delta V fare for YUL<->ICN:

Code:

YMQ DL X/MSP DL SEL Q10.96 596.73VLOGXNMT DL X/DTT DL YMQ Q10.96 596.73VLGGXNMT NUC 1215.38 END ROE 1.368282 XT 19.40CA 26.20SQ 1.31XG 2.61XQ 13.04YC 14.00XY 7.92XA 11.20AY 21.10BP 337.40YR 4.50XF DTW4.50

In this instance, the YR fee is $337. While I am admittedly a novice at this travel geek thing, this really surprised me. I would have thought that the YR fee on YUL-ICN would be either the same (if DL set these fees based on regions; i.e. US to East Asia) or higher (if it was based on distance, which would make sense since it is ostensibly related to fuel costs). Switching to YUL-PSN (Busan), I see that it has the same YR as ICN does:

Code:

YMQ DL X/DTT DL X/SEL DL PUS Q10.96 648.39VLOGXNMT DL X/SEL DL X/MSP DL YMQ Q10.96 648.39VLOGXNMT NUC 1318.70 END ROE 1.368282 XT 19.40CA 26.20SQ 1.31XG 2.61XQ 13.04YC 14.00XY 7.92XA 11.20AY 21.10BP 6.80DA 337.40YR 4.50XF MSP4.50

So, it seems to me that YR is calculated on a country pair-by-country pair basis. Is that correct? Maybe it's just correct for Delta? Additionally, the fact that SK's YR is lower than Japan's is surprising, since SK is farther away from NA; in general, does one have to assume that the YR fees for any given country pair will be random?

I've used Delta as an example here (as it and other SkyTeam airlines are what I am particularly interested in), but feel free to discuss policies for any airline in this thread.