Impact of the sample rate of the emitting signal : communication between HackRF One and USRP NI 2921
The HackRF doesn't support those sampling rates. From reading the firmware over reading libhackrf to reading the gr-osmocom hackrf_sink_c: 

The set of supported sample rates is very limited:

osmosdr::meta_range_t hackrf_sink_c::get_sample_rates()
{
  osmosdr::meta_range_t range;

  /* we only add integer rates here because of better phase noise performance.
   * the user is allowed to request arbitrary (fractional) rates within these
   * boundaries. */

  range += osmosdr::range_t( 8e6 );
  range += osmosdr::range_t( 10e6 );
  range += osmosdr::range_t( 12.5e6 );
  range += osmosdr::range_t( 16e6 );
  range += osmosdr::range_t( 20e6 ); /* confirmed to work on fast machines */

  return range;
}


So, use 8 MS/s.

The hackrf is instructed to use a different sampling rate than you think it uses, which obviously "scales" the spectrum.