I will try to explain as best I can.
Lumineon's power targets Benched Pokemon with "maximum HP of 100 or more."
The phrase "maximum HP of 100 or more" refers to the HP printed on every card. For example, Lumineon's maximum HP is 80. Eighty hit points is the maximum that Lumineon can have. You can't have 90 HP, because Lumineon's maximum is 80. That's the value printed on the card. It's the most damage a Pokemon can take before being Knocked Out.
Cards that add to a Pokemon's maximum HP are also counted when figuring a Pokemon's maximum HP. Let's say your opponent has Rocket's Sneasel ex on his Bench. Rocket's Sneasel ex has 90 HP. That is the maximum HP for that Pokemon. Since that number is not 100, Lumineon can't target it with the power. Lumineon can only target Pokemon with maximum HP of 100 or more, and Rocket's Sneasel ex has 90 HP.
Now, let's say your opponent also has Unown E (MT) on his Bench, and he uses Unown E's power Equip to attach it to Rocket's Sneasel ex. While attached to a Pokemon, Unown E gives that Pokemon +10 HP. This means that you add 10 HP to whatever the maximum value is for that Pokemon.
Effects done to a Pokemon's maximum HP such as this must also be considered when trying to figure out which is a valid target.
Now that Rocket's Sneasel ex has Unown E attached to it, its maximum HP is now 100 - The number printed on Rocket's Sneasel ex is 90. Unown E being attached adds 10 more to the maximum HP as long as it's attached, so now it's 100.
90 + 10 = 100.
Now that Rocket's Sneasel ex has a maximum HP of 100, it can be targeted with Lure Ring.
Now let's say you lure out that Rocket's Sneasel ex, because you can now, as I explained above. You hit it for 30 and now it's his turn.
Let's say your opponent retreats Rocket's Sneasel ex and plays Windstorm to discard the Unown E attached to his Rocket's Sneasel ex.
Rocket's Sneasel ex only has a maximum HP of 90 again. It had 100 HP with the Unown E attached, but since it was removed, Rocket's Sneasel ex doesn't get that bonus anymore.
100 - 10 = 90.
Now that Rocket's Sneasel ex only has a maximum HP of 90 again, you wouldn't be able to target it with Lure Ring. You'd have to pick someone else.
Rocket's Hideout (+20 HP to Dark or Rocket's Pokemon), Low Pressure System (+10 HP to [G] and [L] Pokemon), and Energy Root (+20 HP to whatever it's attached to) are also cards that affect a Pokemon's maximum HP.
Now here's some easy examples. Tyranitar MT has 140 HP. The number printed on the card is 140. That's the maximum HP. That number meets the condition of "100 or more," so Lumineon can target it. Easy, right?
Happiny MT has 60 HP. Lumineon cannot target Happiny because the maximum HP for Happiny is not 100 or more. It's 60.
And let's not forget about maximum HP modifiers. Masquerain MT is a great example with only 70 HP. Can Lumineon target it? No. Why? Because the maximum HP for Masquerain is 70.
But what if, during your opponent's next turn, he attaches Energy Root to Masquerain and plays Low Pressure System?
Base 70 HP + 20 from Energy Root + 10 from Low Pressure System = 100 HP
Masquerain has maximum HP of 100 with these cards in play. Can Lumineon target Masquerain now? Yes. Why? Because even though the HP printed on Masquerain's card is 70, the effects made to the maximum HP (+20 from Energy Root and +10 for Low Pressure System) increase it from 70 to 100, which means it meets the requirement and can now be targeted.
Let's say you play a Stadium next turn and get rid of Low Pressure System. Assuming Masquerain still has Energy Root attached, Masquerain's maximum HP is now 90. Can Lumineon target it now? No. Why? Because Lumineon can only target cards with maximum HP of 100 or more, and 90 is not "100 or more."
Do you understand now?