Nargott
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Increasing your speed by a factor of 2 and decreasing the opponent's speed by a factor of 2 are similar things only under 3 conditions simultaneously:
1) It is applied to all troops, not to a single target/area.
2) It is applied for a long/unlimited time.
3) Heroes do not participate in the battle after that (do not cast spells).
Since these conditions are not met, the multiplication model works less effectively. It's like physics with an ideal gas and a real gas; under certain conditions (large numbers), the formulas for an ideal gas are not suitable due to error.
Illustration:
Multiplying the initiative of one unit with retaliation to infinity = practically an automatic win; but zeroing out the initiative of the enemy unit even until the end of the battle (blindness) is by no means a win.