How is planetary trade income determined?

I've observed some strange patterns in trade income in Slipways (looking at the info screen for a planet, observe the value indicated for 'trade income'). While playing on Tough difficulty,

• Most planets appear to earn you 4 credits per trade they send out.
• When a planet is struggling or sending to a struggling planet, they only appear to award 3 credits
• Prosperous planets seem to award between 4-5 credits per trade.

While, when playing on Challenging;

• Most planets now earn 5 credits per trade sent out
• But now some successful planets award 6 credits for some of their trades as well.
• Prosperous planets can even award 7, but this is rarer than before.

What is happening?

Trade income calculations appear to round multiple times in multiple different ways. There's also a hidden multiplier, but it acts diffrently from the game's other income multipliers. This makes the actual trade income appear to be mysteriously inconsistent. Denote the trade income of a colony as T(c).

T(c) = round((Σ(floor( b * (e(I) + e(E) + 100%)))) * (100% + M))
Where: 
M = income bonuses from technology/perks
b = base income, depends on difficulty
e = export multiplier for
    I = importer
    E = exporter

The base income per difficulty is the following:

Forgiving       7
Reasonable      6
Challenging     5
Tough           4

The income bonuses from technology/perks are additive. So a planet with Machine interfacing +30% and Economic Zones +60% will have M = +90%

The export multiplier e(I) + e(E) depends on the state of the importing and exporting colony. It's the sum of the percentages applied as a multiplier. These are the following:

struggling    -25%
established     0%
successful    +10%
prosperous    +20%
legendary     +40%

So, for example, a Successful planet sending something to a Struggling planet would have a 10 - 25 = -15% modifier, which results in an income contribution of 5 / 5 / 4 / 3 credits on each difficulty, respectively.