why does my video bitrate change slightly when shortening?

[chsgrits]

I have a VHS capture that I used Lagarith lossless compression set at YUY2 mode in Amarec. I shortened the video using the same codec in virtualdub2 and set the compression and mode to the same but with pixel format set to YUV422, which isn't an option in Amarec, expecting mediainfo to display the same parameters, but they are slightly different. Shouldn't the bitrate be the same?

source:

Video
ID : 0
Format : Lagarith
Codec ID : LAGS
Duration : 1 h 4 min
Bit rate : 47.7 Mb/s
Width : 720 pixels
Height : 480 pixels
Display aspect ratio : 3:2
Frame rate : 29.970 (30000/1001) FPS
Standard : NTSC
Bits/(Pixel*Frame) : 4.603
Stream size : 21.5 GiB (93%)

trimmed video:

Video
ID : 0
Format : Lagarith
Codec ID : LAGS
Duration : 46 min 49 s
Bit rate : 50.4 Mb/s
Width : 720 pixels
Height : 480 pixels
Display aspect ratio : 3:2
Frame rate : 29.970 (30000/1001) FPS
Standard : NTSC
Color space : YUV
Chroma subsampling : 4:2:2
Bit depth : 8 bits
Bits/(Pixel*Frame) : 4.868
Stream size : 16.5 GiB (97%)

[davexnet]

The codec is lossless not uncompressed, so for any size produced you have to take into account the complexity
of the video