Gap Connection Event Length changes when using different smartphones

First about my setup: custom devices with NRF52832 chips (SoftDevice 132 v.4.0.2), android smartphones Nexus 5x and 6p.

I'm trying to connect as many BLE devices (peripherals) as it is possible to a smartphone (central) within the fixed connection interval of 30 ms. The GAP event length is set to 2 (in 1.25 => 2.5ms) for each BLE device. Therefore the theoretical amount of maximum central links with this CI is 12 (2.5 *12 = 30). So here is what I saw using Nexus 5x.

D0 and D1 - radio ON signals of first and second BLE devices

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The event length between two central links is set to 2.5 ms - as I expected. After that I tried with Nexus 6p:

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The event length changed to 3.7 (3.75) ms.

Is there any way to overcome this problem or it will always vary on different smartphones?

Note that you can't set the GAP event length using Android BLE stack.