You're measuring the time from the end of frame#1 to the beginning of frame #2.

A = get the time.
...spend a while doing stuff...
B = get the time
...loop for next frame...
C = get the time
time spent doing stuff previously = C-B  (wrong)
...spend a while doing stuff...
etc

You really want to be measuring the time from the start of frame#1 to the start of frame#2.

A = get the time.
...spend a while doing stuff...
...loop for next frame...
B = get the time
time spent doing stuff previously = B-A
...spend a while doing stuff...
etc

You're measuring the time from the end of frame#1 to the beginning of frame #2.

A = get the time.
...spend a while doing stuff...
B = get the time
...next frame...
C = get the time
time spent doing stuff previously = C-B  (wrong)

You really want to be measuring the time from the start of frame#1 to the start of frame#2.

A = get the time.
...spend a while doing stuff...
...next frame...
B = get the time
time spent doing stuff previously = B-A

You're measuring the time from the end of frame#1 to the beginning of frame #2.

A = get the time.
...spend a while doing stuff...
B = get the time
...next frame...
C = get the time
time spent doing stuff previously = C-B  (actually its C-A!)

You really want to be measuring the time from the start of frame#1 to the start of frame#2.

A = get the time.
...spend a while doing stuff...
...next frame...
B = get the time
time spent doing stuff previously = B-A

You're measuring the time from the end of frame#1 to the beginning of frame #2.

A = get the time.
...spend a while doing stuff...
B = what's the time
...next frame...
C = get the time
time spent doing stuff previously = C-B  (actually its C-A!)

You really want to be measuring the time from the start of frame#1 to the start of frame#2.

A = get the time.
...spend a while doing stuff...
...next frame...
B = get the time
time spent doing stuff previously = B-A

You're measuring the time from the end of frame#1 to the beginning of frame #2.

A = what's the time.
...spend a while doing stuff...
B = what's the time
...next frame...
C = what's the time
time spent doing stuff = C-B

You really want to be measuring the time from the start of frame#1 to the start of frame#2.

A = what's the time.
...spend a while doing stuff...
...next frame...
B = what's the time
time spent doing stuff = B-A