Simple question:
When the DFL can be below 1.0?

Just read this sentence in DNV-RP-D101 clause 2.2.6,

"Dynamic load factors that are selected from figures or charts depending on the load duration and natural frequency of the piping may be below 1.0, but should be chosen as high as 1.5-2.0, when the natural frequencies and load duration time are unknown, or otherwise when DLF figures for the actual component/or system being analysed are not established."

Simple question merits simple answer:

When the dynamic response of the system to the load in question results in stresses lower than the load would be if it were applied statically.

Even Michael answered, you may find interesting to see some examples in the book "Introduction to Structural Dynamics, J. M. Biggs, McGraw-Hill Book Co., 1964", which is referred by Figure II-3.5.1.3-2 Dynamic Load Factors for Open Discharge System, Appendix II of B31.1 (search the book on internet).

As a rule, in case a structure is subject to a short duration load whose duration is much lower than the natural period of the structure, then the dynamic response will be lower than a static response using the same load. See the figures 2.6 and 2.7 in the book, for td/T<0.1.