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Let me see if I can help you...
Trim is the physical page size... it is the point (within a degree of error) that the page will be cut AFTER it is printed.
Bleed is generally 1/8" or 1/4" area OUTSIDE of the trim. It's purpose is to ensure that in areas where ink (or color) is to touch the physical edge of the page, that a small amount of 'overprint' compensates for the margin of error during TRIM.
Lets say you are printing a dark background onto a white stock (paper). If you start with a paper that is 12 X 14 and want to trim a page of 8.5 x 11, the printed image will be centered on the 12 x 14 page leaving a lot of white space around the edges. Now you need to TRIM that page down to 8.5 x 11. If your printed image was EXACTLY 8.5 x 11, the unavoidable degree of error in the TRIM process would leave a slight white margin along one or more sides of the page.
This error of margin is caused by any number of things... blade wobble on older cutters, even the slightest skew in the paper feed, even paper expansion due to humidity.
The solution is to print MORE of the image than is actually required, so that when you do trim to 8.5 x 11, you trim of a slight portion of the outer edge of the image, but there is no white margin anywhere.
You'll find super high volume printers like Magazines and Newspapers have wider bleed because there equipment feeds and prints so fast that it cannot be 10,000's of an inch accurate every time.
When you get into more custom and high end print jobs, the equipment runs at lower speeds and is typically much more accurate, however, event these print houses generally like a margin of error 'just in case'.
Now, as to why your TRIM size and IMAGE AREA are 1.25" I can only guess... since it's a magazine, they will have a binding edge (usually what is called 'perfect binding' where there is a spine and no staple or stitch). This makes a portion of the actual page unreadable because it's tucked into the binding. Then most magazines place there page numbers and other stuff in the bottom 1/4" of the page...
I hope that answers your questions...
Mike.
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