It doesn't say so anywhere I've found, but the lower hull may be a modular or "swapable" section. Since its such a small ship and its scientific duties would be widely varied, it seems reasonable that the lower hulls can be swapped out for mission-specific platforms. This would increase their flexibility and reduce the costs of building several whole ships, and having them separated from the hull would make the process a lot faster and simpler.
For example, if Starfleet expected to need (over a 5-year period) three small ships for planetary surveys, three for space phenomena studies, and one electronic warfare platform for the fleet, they'd need to build seven ships. Or they could build three ships plus four additional lower hulls at a third to half the price that could be stored until needed.
Keep in mind that, as the Oberths got older, more were seen as specialist ships, not always science vessels. By the middle of TNG and the beginning of DS9, Oberths were being used as personnel transports, high-priority freighters, couriers, tech support ships, etc. The Cochrane was Admiral Setie's personal VIP transport for awhile, then continued as a long-range personnel carrier. The Pegasus was used to test experimental equipment, such as a Federation cloaking device and the Type-X phasers. I've even seen an image of one without the lower hull being used as shipyard's tug/crane. (BTW, the same thing happened to the real world BB-5 Kearsarge when it was converted to Crane Ship No.1 in 1920.)
The modular idea isn't without precedent. Bridges, sensor pallets, and other external equipment are known to be "plug-and-play" modules. One of the theoretical replacements for the Galaxy-class included huge pie-shaped saucer sections that could be swapped out for mission-specific needs. So again, its just a theory, but it makes a lot of things make more sense.