I'm working on an article about the tech levels in Red Alert. There are only 10 levels that you can choose from in the game, but there are some units and structures which have a higher tech level than 10. Why is this so?

Tech Level 1
Ore Truck
Rifle Infantry
Grenadier
Refinery
Silo
Power Plant
Barracks (Allied)
Barracks (Soviet)
Fake Construction Yard

Tech Level 2
Rocket Soldier
Medic
Pillbox
Flame Tower
Sandbag Wall
Wire Fence

Tech Level 3
Ranger
Mine Layer
Transport
Attack Dog
War Factory
Naval Yard
Sub Pen
Camo Pillbox
Radar Dome
Kennel
Service Depot
Fake War Factory
Fake Naval Yard
Fake Radar Dome

Tech Level 4
V2 Rocket
Light Tank
Heavy Tank
Turret

Tech Level 5
APC
Submarine
Gunboat
Engineer
Yak
AA Gun
Airfield

Tech Level 6
Medium Tank
Flamethrower
Spy
Tech Center (Soviet)

Tech Level 7
Destroyer
Tesla Coil

Tech Level 8
Artillery
Advanced Power Plant
Concrete Wall

Tech Level 9
Longbow
Hind
Helipad
SAM Site

Tech Level 10
Mammoth Tank
Cruiser
Mig
Gap Generator

Tech Level 11
Mobile Gap Generator
MCV
Thief
Tanya
Chinook
Tech Center (Allied)

Tech Level 12
Mobile Radar Jammer
Iron Curtain
Chronosphere

Tech Level 13
Missile Silo