There are two ways that jumps are defined - in seconds and in percentages.
Kodi decides whether to use a jump in seconds or percentage depending on the length of the video. In general this means that for a long video the jumps will be in fixed numbers of seconds/minutes, but for shorter videos the jump size will be reduced and be a percentage of the total video length, and get progressively shorter for shorter videos - this is particularly apparent when you play something like a 2 minute movie trailer where a small step forward (normally 30 seconds) will only jump about 2 seconds.
I’m far from a video expert but the actual seek time changes due to iframes (I think they’re called) when seeking the player jumps to the nearest full frame at the desired point which may be several seconds from target