I understand your point, but you are still wrong. Even if TIM earns three times the XP of BOB its level will not be 3 times higher, only its total XP. XP needed for next level increases, starting with 0, 1000, 2000, 3000…
Here is a level comparison between TIM and BOB. It shows the level of TIM(right) when BOB gets to a specific level (left):
2 - 3
3 - 4
4 - 5
5 - 7
6 - 8
7 - 9
8 - 11
9 - 13
10 - 14
11 - 15
12 - 16
13 - 18
14 - 19
From the data I would say that TIM has about a 33% increased level compared to BOB but will also fight enemies that are 33% higher level.
In addition the gap between a nether and a normal creature will not increase in relative terms. If your nether’s stat are three times higher than that of a normal creature at level 1 they will be three times higher when both creatures are level 100. The absolute gap will increase. What does that mean for a certain realm level?
Lets say BOB is 25% more powerful than TIM at level 1 (which I consider low if TIM really earns XP at 300% rate) and TIM has three times the power of a normal creature (putting BOB at 3.75 times). Lets say the base creature for TIM and BOB is a Brownie Brute with 40 attack. Their attack at any point in time would be
TIM: 1.33LevelBOB0.2340 + 340 (1.33=level advantage of TIM, 0.2=factor of stat gain per level, 340=TIM’s power at level 1)
TIM: 31,92LevelBOB + 120
BOB: 30LevelBOB + 150
So you are right that TIM will become more powerful than BOB at a certain point (past level 15). However lets have a look at the enemies they are fighting.
I made a little test run with a group of level 1 creatures in realm level 10 to determine how realm level affects the level difference between player creatures and enemy creatures. From what I can tell (and Zack can correct me if I am wrong) it appears to be an absolute level difference of about 24 level meaning in realm 10 an enemy creature has about 4.8*base more stat compared to a player creature. An enemy Brownie Brute would have 192 more attack than a player’s.
When BOB is level 100 he will be fighting level 124 enemies in realm level 10 while TIM is level 133 and will be fighting level 157 enemies.
TIM will have 3312 attack, an enemy Brownie would have 1296 attack
BOB will have 3150 attack, an enemy Brownie would have 1032 attack
TIM would have 2016 more attack than the enemy or 255%
BOB would have 2118 more attack than the enemy or 305%
BOB is more powerful than his opponent both in relative and absolute terms and will be able to go deeper!