If you use constant bitrate as rate control, which is used either if you use (use stream encoder) as encoder or if you explicitly choose CBR as rate control in advanced output mode, OBS is able to compute the exact buffer size by multiplying the seconds with the bitrate and divide by 8 (bit per byte). If you don't use CBR but a quality based rate control, bitrate is variable so an exact buffer size cannot be predicted. May be OBS is estimating buffer size according to the given time, and it gets 25% wrong, or you actually use constant bitrate and the calculation is 25% off due to some unknown reason. I don't know what happens.