Question regarding demo update rate

In the config file under the demo section, It is said for _C.DEMO.BUFFER_SIZE = 0:

"_Number of overlapping frames between 2 consecutive clips.
Increase this number for more frequent action predictions.
The number of overlapping frames cannot be larger than
half of the sequence length `cfg.DATA.NUM_FRAMES * cfg.DATA.SAMPLING_RATE_"

Why does the number of overlapping frames is bounded to be **half of the sequence length**? Isn't it bounded by the **sequence length -1?**