Audio limited or compressed by default iMac 27 mid 2015

When playing a beat in Logic Pro X 10.5 (and mp3’s and other kind of audio files) using built in speakers the volume seems to start at a level and then after some seconds it reaches the right level (about 3-4 dB louder). It sounds like its a limiter or compressor that slowly lets the volume up to the right level. Using a soundcard like Focusrite Scarlett and earphones this is not happening. It doesn’t seem to be any plugin in Logic as this phenomenon happens with bounces from Logic - the mp3 file acts the same way - when starting the file in Quicktime for example its lower in dB than after some seconds when it reaches its right volume. These are small changes but measurable with a dB meter, and it’s annoying. This happends if I start an audio file lets say 2 minutes into it - where I know the volume should be at its peak - the playback starts at a lower dB even in this case (in the Macs speakers). This does not occur when using headphones (directly into the Mac) or to the sound card. What is this?