were I think your house rule is fine you missed something... ecl 1 isn't always a level behind it is 1,000xp behind but at level 7 it is just a blip (level 6 xp is 15,000 level 7 is 21,000 level 8 is 28,000) ecl 2 is 3,000 behind... if the party has 18,001xp or more they are all 6th level even the ecl 1 and 2.)
You use the total level + ECL to figure out where on the level chart you are.
DMG page 172
"Use ECL instead of character level when referring to Table 3-2 Experience and Level-Dependent Benefits in the Player's Handbook to determine how many experience points a monster character needs to reach its next level."
Let's look at an ECL +2 PC vs. a normal one.
Normal: Needs 1000xp to reach level 2.
ECL+2 PC: Needs 3000xp to reach level 2 since he's starting at "level 3."
ECL is 2000 behind.
Normal: Needs 2000 to reach level 3.
ECL: Needs 4000 to reach level 3.
Normal has now earned 3k and ECL has earned 7k to reach the same level. ECL is now 4k behind.
Normal: Needs 3k to reach level 4.
ECL: Needs 5k to reach level 4.
Normal has now earned 6k and ECL has earned 12k. ECL is now 6k behind. And it keeps getting worse.
That was for both to reach level 4 in a white room. In game play since the ECL PC has been higher level the entire time due to ECL, he is earning LESS XP per fight than the non-ECL PC. So by the time the normal PC has earned the 6k and is level 4, the ECL PC as probably only earned 4k and is level 3. This rebalances at higher levels since the ECL PC will eventually be lower level than the normal PC, even after EC is counted.
As the levels go on, the ECL PC falls behind by 2k per level. At level 12 the ECL PC is now more than 24k behind the normal PC. That puts him at level 9, 11 with his +2. He's now a full level lower than the normal PC and his +2 ECL abilities haven't helped much for several levels, so he's really just level 9 while the rest of the group is level 12.
Edit: That's not quite right. Once the party is two real levels higher than the ECL PC, they will earn the same XP forever and he will be 2 levels behind from there onward. The ECL PC can't ever catch up. So when the normal PC hits 20th level, the ECL PC will be 18th.