How is it the same ratio if the screen is taller? Why watch HD movies on something that isn't scaled properly.
Looks like you need a math lesson.
Let's start by defining the word "ratio"
ra·tio[ ráyshō ]ra·tios Plural
NOUN
1. proportional relationship: a proportional relationship between two different numbers or quantities
"The ratio of teachers to students at that school is 1 to 27"
2. mathematics one number divided by another: a quotient of two numbers or expressions arrived at by dividing one by the other
Next, let's look at what the aspect ratio of 720p is:
1280x720 aspect ratio would be the width divided by the height
1280/720 = 1.78 (rounding to nearest hundredth)
Another way to do this is to reduce the fraction to it's lowest terms
1280/720 = 16/9 (using 80 as the divisor)
Now, let's look at the aspect ration of the 4" iPhone screen:
1136/640 = 1.78 (rounding to the nearest hundredth)
Or reducing the fraction
1136/640 = 16/9.01 (using 71 as the divisor...this one isn't perfect)
So as you can see, both 1280x720 and 1136x640 have a (aspect) ratio of 1.78 or 16/9.