I am pretty sure there is some truth to that or at least I personally believe there is. Not sure how it exactly works but it seems everyone is hit with soft reset at start of the season. But the longer you wait the less of a reset your hit with. I think its tied to player rank distribution.
At the start of the season challenger - diamond players are kicked out of those divisions and can only be placed up to plat 1. Well this kicks plat players out of plat, gold out of gold. etc. etc. So if you look at the start of the season at player rank distributions and compare it to the end of the season you will see 2 things. 1. A lot less players currently ranked, since not everyone does their placements right away. 2. Rank distributions are different.
For example. Right now 36% of the rank population is gold V or higher. So a player has to be within the top 36% of accounts to at least be in gold V.
http://na.op.gg/statistics/tier/ But if you look at these stats the first few weeks or first couple months of you will see the numbers change. First week no one will be in challenger/master, it takes them time to climb back up. You will also see to be gold V you have to be within something like the top 25% of the population rather than 36%. I actually saved my information from the start of season 6 and looks like hitting gold V 0 lp on April 2, 2016 = top 24.62% of ranked population.
Well as the season goes on more players play their placements and increase the player pool. Simultaneously you will have people already placed climbing the ladder. So what my theory is, based on what I have seen in the past, is the longer you wait to do your placements and the more people who enter the ladder the less of a rank reset your hit with. Like in your example you went 10-0 in placements but were hit with bronze 5. This makes me assume your were bronze in season 5. But say you wait like 4-5 months to do your placements. If you then went 10-0 or even like 8-2 you probably would have gotten more like bronze 3-4 (depending on who you play vs and your prior season rank) rather than bronze 5.
Its even seen more in like silver I think or gold-plat. I noticed in past seasons I would drop a few divisions if I did placements at the start of the season. But if I waited instead of losing say 4-5 divisions with a similar record I might lose 2-3 instead. But after a few months more people are playing and the rank populations are closer to what they normally are.
So I do think starting rank does depend a little on when you do your placements. The trade off being if you wait you might have a slightly higher season starting rank than your would if you did placements right away, but you also have less time to grind the ladder compared to your peers who did their placements right away.
