Where max-wiidth (@media) css come from?

Where max-wiidth (@media) css come from?

I have this problem, there are 2 pages, that some part of the code is
repeated, to this part of code, I apply this css:
@media (max-width: 767px){
.sliderContent{
width:100%;
}
}
When I check one page in iphone, it loads this css, but when I check the
other does not use this style.
I have added an alert with the screen size in both pages, and both have
the same width.
Do you know where the max-width is taken? that could be making the
difference between the two pages?
Can the html somehow affect the size of the min-width??
Thanks in advance