The best answer / explanation for this can be found in the datasheet:
PFM Control Scheme
The MAX17220/2/3/4/5 utilizes a fixed on-time, currentlimited, pulse-frequency-modulation (PFM) control scheme that allows ultra-low quiescent current and high efficiency over a wide output current range. The inductor current is limited by the 0.225A/0.5A/1A N-channel current limit or by the 300ns switch maximum on-time. During each on cycle, either the maximum on-time or the maximum current limit is reached before the off-time of the cycle begins. The MAX17220/2/3/4/5's PFM control scheme allows for both continuous conduction mode (CCM) or discontinuous conduction mode (DCM). When the error comparator senses that the output has fallen below the regulation threshold, another cycle begins. See the MAX17220/2/3/4/5 simplified functional diagram.
The MAX17220/2/3/4/5 automatically switches between the ULPM, low-power mode (LPM) and high-power mode (HPM), depending on the load current. Figure 4 and Figure 5 show typical waveforms while in each mode. The output voltage, by design, is biased 2.5% higher while in ULPM so that it can more easily weather a future large load transient. ULPM is used when the system is in standby or an ultra-low-power state. LPM and HPM are useful for sensitive sensor measurements or during wireless communications for medium output currents and large output currents respectively. The user can calculate the value of the load current where ULPM transitions to LPM using the equation below. For example, for VIN = 1.5V, VOUT = 3V and L = 2.2µH, the UPLM to LPM transition current happens at approximately 1.49mA and a no-load frequency of 11.5Hz. The MAX17220/2/3/4/5 enters HPM when the inductor current transitions from DCM to CCM.
(Figures 4 and 5, ULPM, LPM, and HPM Waveforms, in the datasheet offer a great visual explanation of these transitions, and ultimately the answer to the question.)