Why is packet.size not honoured for the buffer here?

[htool]

While investigating why canboat and canboatjs are treating a MOB pgn Route/Navigation route differently I noticed this:

canboatjs/lib/fromPgn.js

Line 147 in f541d1b

var newSize = packet.size + FASTPACKET_BUCKET_N_SIZE;

Observe the different behaviour.

Here's a full 129285 packet coming from my MFD:

2021-11-18-13:12:35.444,7,129285,6,255,8,c0,3a,ff,ff,02,00,ff,ff
2021-11-18-13:12:35.445,7,129285,6,255,8,c1,ff,ff,e7,03,01,00,83
2021-11-18-13:12:35.446,7,129285,6,255,8,c2,00,00,03,01,00,ff,ff
2021-11-18-13:12:35.446,7,129285,6,255,8,c3,ff,7f,ff,ff,ff,7f,01
2021-11-18-13:12:35.446,7,129285,6,255,8,c4,00,16,01,4d,4f,42,20
2021-11-18-13:12:35.447,7,129285,6,255,8,c5,41,49,53,20,53,41,52
2021-11-18-13:12:35.447,7,129285,6,255,8,c6,54,2d,39,37,32,37,37
2021-11-18-13:12:35.448,7,129285,6,255,8,c7,37,33,e4,c9,44,1f,29
2021-11-18-13:12:35.467,7,129285,6,255,8,c8,be,01,03,ff,ff,ff,ff

Canboat's analyser says it sees 2 items:
2021-11-18-13:12:35.467 7 6 255 129285 Navigation - Route/WP Information: Start RPS# = Unknown; nItems = 2; Database ID = Unknown; Route ID = Unknown; Navigation direction in route = Unknown; Supplementary Route/WP data available = 1; Route Name = ; WP ID 1 = 0; WP Name 1 = ; WP Latitude 1 = Unknown; WP Longitude 1 = Unknown; WP ID 2 = 1; WP Name 2 = MOB AIS SART-9727773; WP Latitude 2 = 52.4601828; WP Longitude 2 = 5.0445865

Canboatjs says it finds 3 items, of which the 3rd seems to be a misread of the stuffing 0xff packets:
canboatjs:fromPgn parsed pgn {"canId":502859014,"prio":7,"src":6,"dst":255,"pgn":129285,"timestamp":"2022-01-11T20:10:37.492Z","input":[],"fields":{"nItems":2,"Supplementary Route/WP data available":1,"Reserved1":"14","Route Name":"\u0000","Reserved2":"131","list":[{"WP ID":0,"WP Name":"\u0000"},{"WP ID":1,"WP Name":"MOB AIS SART-9727773","WP Latitude":52.4601828,"WP Longitude":5.0445865},{"WP Name":"","WP Latitude":0.0000255}]},"description":"Navigation - Route/WP Information"} +0ms
If I remove the overwrite of the packet.size, it works fine:
canboatjs:fromPgn parsed pgn {"canId":502859014,"prio":7,"src":6,"dst":255,"pgn":129285,"timestamp":"2022-01-11T20:11:24.837Z","input":[],"fields":{"nItems":2,"Supplementary Route/WP data available":1,"Reserved1":"14","Route Name":"\u0000","Reserved2":"131","list":[{"WP ID":0,"WP Name":"\u0000"},{"WP ID":1,"WP Name":"MOB AIS SART-9727773","WP Latitude":52.4601828,"WP Longitude":5.0445865}]},"description":"Navigation - Route/WP Information"} +1ms
So I'm wondering why the given (and correct) packet.size is not honoured and used for the buffer...
Changing to:
var newSize = packet.size
makes it work as expected.