I have 4 possible weapons in ToT as a hunter.
One drops off Tortos, a 20 item loot table. Assume they drop equally (we already know this isn't true and less-represented items that are for singular classes, like bows for hunters, tend to have lower drop rate than int cloth, for example). That would give the item a 5% drop rate.
In 10 man, you have a (1- (1/20)) * (1- (1/19)) chance it won't drop in a given week, which, as someone pointed out, is a 90% chance it won't drop, week to week. Over 10 weeks, that chance of it never dropping is 0.9^10, or 35% chance it will never drop in 10 weeks.
In 25 man, on a given week you can up to 2 repeats of an item I believe, so you have a (0.95^2) * (0.948^2) * (0.945^2) chance, or 72.4% best case for it to not drop (worst case is 6 unique items, or (0.95^6) = 73.5% to not drop in a given week). Over 10 weeks, this gives a range of 3.95%-4.6% chance for the item to not drop. So, unless you have 10 hunters in your raid to our 1, your hunters will get this bow long before I do.
But surely I'll get one weapon, right? No one knows where two of the bows drop from, one is rumored to be a trash drop, which means, guess what, no heroic version for me. Another is conjectured to be off Iron Qon.
1/20 chance on Tortos x 2 as non-tier
1/20 chance on Durumu x 2 as non-tier
1/11 chance on Iron Qon (speculation) x 1 as tier boss.
As above, I have a 90% chance on both those first bosses to never see the bow in a given week in a 10 man, and 91% chance to not see it off Iron Qon. That means I have a 73.7% chance not to see ANY ranged weapon in a given week (still higher than the individual chance to not see a weapon off any specific boss in 25 man). Over 10 weeks, that means I have a 4.72% chance of not seeing a weapon (1 in 20 almost, and that's enough RNG to make me cringe).
In 25 man, I have, as above, a 73.5% worst case chance to not see a weapon off those first two bosses, but I have a (0.909^4) = 68.2% chance not to see the weapon off Iron Qon (remember, 25 man drops 2 tier pieces, 4 non-tier pieces). That means in ONE WEEK of 25 man, I have a (.735^2) * .682 = 36.8% chance of not seeing a ranged weapon in a given week. Already, in a given week, you're seeing more than a double chance of getting a ranged weapon than a 10 man hunter. Over 10 weeks, this is a 0.0045% (yes, that's 0.000045) that you will not see a ranged weapon. The chance of seeing 3 weapons over 10 weeks is 99.995%^3, or 99.986%, and no 25 man I know has more than 3 hunters.
Surely the math is fairly clear about how different the the gearing is. Factor in less role diversity in 10 man and thus a higher chance of sharding something, and it's even worse.