Your screenshot shows the 100% being selected and Apply button greyd out, not functioning. This means that you have already 100% selected and in use; the Apply button is visible and functioning only when when you change a value, not visible and functioning when the selected value (in this case 100%) is already set.
My Computer
At a glance
Windows 10 Pro x64 EN-GB1.6 GHz Intel Core i7-720QM Processor6 GBATI Mobility Radeon HD 5850 Graphics
Computer type
Laptop
Computer Manufacturer/Model Number
HP ENVY 17-1150eg
OS
Windows 10 Pro x64 EN-GB
CPU
1.6 GHz Intel Core i7-720QM Processor
Memory
6 GB
Graphics Card(s)
ATI Mobility Radeon HD 5850 Graphics
Sound Card
Beats sound system with integrated subwoofer
Monitor(s) Displays
17" laptop display, 22" LED and 32" Full HD TV through HDMI
Screen Resolution
1600*900 (1), 1920*1080 (2&3)
Hard Drives
Internal: 2 x 500 GB SATA Hard Disk Drive 7200 rpm
External: 2TB for backups, 3TB USB3 network drive for media
Cooling
As Envy runs a bit warm, I have it on a Cooler Master pad
My screenshot shows the system default is 125% but I set it to 100%. I want to change the default to 100%. The system has 125% as the default not 100%.
The 125% default is what Widows 7 recommends. You don't have to use it if you don't want to.
You have chosen 100% and Windows 7 will honor that choice and use that choice.
You are worried about the word default. Forget about it and be happy.
My Computer
At a glance
Windows 10 Pro. 64/ version 1709 Windows 7 Pr...Intel i7-6800K @ 4.3Corsair Platinum 16 gig @2400EVGA GTX 1070 OC
Computer type
PC/Desktop
Computer Manufacturer/Model Number
Home made Desktop
OS
Windows 10 Pro. 64/ version 1709 Windows 7 Pro/64
CPU
Intel i7-6800K @ 4.3
Motherboard
ASUS X-99 Deluxe II
Memory
Corsair Platinum 16 gig @2400
Graphics Card(s)
EVGA GTX 1070 OC
Monitor(s) Displays
Asus 27" LED LCD/VE278Q
Screen Resolution
1920-1080 or 1280-720 HDMI
Hard Drives
INTEL SSD 730-240 Gb Sata 3.0/
PSU
EVGA Platium 1200W
Case
Phanteks Luxe Tempered Glass 8 fans/ one radiator
Cooling
XSPC/ Water Cooled CPU
Keyboard
Das 4 Professional
Mouse
Logitech M705/MX Anywhere 2-S
Internet Speed
100 mbits
Antivirus
Microsoft Security Essentials/ Malwarebytes Premium 3.0/ SAS
Browser
I.E. 11 default/Firefox/ ISP Time Warner Cable/Spectrum
Other Info
LG BluRay Burner/
Sound system-KLipsch-THX/
Icy Dock ssd Hot Swap bays.
I'm not being sarcastic when i say this but that is the quote of the week for me
It reminds me of another quote by Barney Stinson from How i Met Your Mother;
My Computer
At a glance
Windows 8.1 Pro x64Intel Core i5-2500K @ 3.30GHz - S11558GB Corsair DDR3 XMS3, PC3-12800NVIDIA GeForce GTX 650