I think you misunderstand what the arc above a Pokémon means, and it's causing you some confusion.
The arc above a Pokémon represents the "power up potential". A trainer can only power up a Pokémon to be 1.5 levels above their current trainer level. At level 25, you can only have Pokémon that are level 26.5. Each time you power up a Pokémon, their level increases by 0.5.
Your second Jolteon might be at level 26, which means you can only power it up once. While the first Jolteon might only be at level 23, and can be powered up 7 more times before reaching the maximum for your trainer level.
If you look at the amount of stardust and candies required to power up each Pokémon, it will give you an idea of what level the Pokémon is. If the second Jolteon is level 25, it should require 4000 stardust and 3 candies. Level 26 would require 4000 stardust and 4 candies. If the first Jolteon is between level 21 and 23, it will cost 3000 stardust and 3 candies. Between level 23 and 25, requires 3500 stardust and 3 candies.
As an example, take a look at these Pidgeot.
Notice that these first two both require 3500 stardust and 3 candies to power up, and that the arc meter above them is filled to the same position. Both of these Pidgeot are at the same level, so their arcs are also the same. However, since they have different Individual Values (IVs), their CP is different.
If you look at this next Pidgeot.
You'll notice that it only requires 3000 stardust to power up, and the arc above it is not as full. That's because this Pidgeot is at a lower level than the others.
In contrast, this level 2 Machop only requires 200 stardust to power up, and has a long way to go before it reaches max level for my trainer level.
You'll notice that the arc above it is not very full, since it's at quite a low level.
So as you can see, the arc above a Pokémon only shows the level of that Pokémon. It does not directly correspond to the CP of the Pokémon, though does play a part in calculating it.
If I were to power up Mike to the same level as the other Pidgeot, it may or may not have a higher CP. Whether or not it does, depends on the IVs of the Pokémon. Keep in mind, however. Even if you power both Pokémon up to the same level, their CP will be different. This is where individual values (IVs) come into play.
Every Pokémon has a set of base statistics that are common among all Pokémon of the same species. They also have a hidden set of values, known as **Individual Values (IVs). These individual values are what cause Pokémon of the same species and level, to have differing CP values. The formula for calculating CP takes into account base stats, IVs, and level.
So if you have a lower level Pokémon with higher CP, you can infer that the lower level Pokémon has better IVs. However, with the introduction of the in game appraisal system. There's no need to try and guess how good a Pokémon is, since your team leader tells you which Pokémon are good.