Hello Doctor,
'twas ever so.
The magnification numbers are not very accurate. The actual size of the screen depends on the magnification you set, the physical size of the screen, and probably other factors as well.
On this machine—a 15" diagonal MBPro with retina, 100% shows a US Letter (8 1/2") width page in approximately 5 1/2 inches of space. 125% give a closer approximation of 'actual' size, packing those 8.5 inches into just over 7" of screen space. 150% is still 3/16" short of the actual 8.5 inches of the page, but also pushes the bottom third of a page (3 3/4 inches) off the bottom of the screen.
On the other side of the room, Pages '09 (previous version) produces these page widths for the same 8 1/2 x 11 inch US Letter size sheet:
100%: 6 1/8"
125%: 8"
150%: 9 1/8"
If the on screen size is important, you can get a closer match by choosing "Fit width" from the pop-up menu in the View magnification box, then dragging the windo's right boundary right or left to make the working part of the window the exact actual width of your page. On my MBP, that resulted in a 152% figure in the box. YMMV.
Regards,
Barry