target dummies have relatively less seal damage due to the absence of the 10% attack speed buff, 5% spell haste buff (more CS hits), the 8% spell damage buff, 4% physical damage taken debuff, 12% armor debuff (SoC) and the 10% AP buff. this means that on a target dummy, the 4PT10 isn't worth as much. you really need to compare logs versus each other to get a decent comparison.
i'm currently working up a comparison to compare 4PT10-251 with 2X251;2X245 (as in the OP example) with rest in 251 gear. don't have time to finish it right now, but i'll come back to this later.
edit: comparison shows it's a minor difference in stats.
http://www.wowhead.com/compare?items...23:50977:49999
on a setup of 2K STR you have a 34 STR, 32 crit and 6 haste difference. it's pretty close.
it really depends on the quality of your tier pieces and the potency of the remaining gear. better offset pieces mean the bonuses scale better than the raw stats. worse offset pieces mean the raw stats are a bigger influence.