Your slides are most likely duplicates. The white borders are where some light leaked through the dark borders surrounding the image in the original film. Duplication is performed by bringing the unexposed slide film's emulsion directly into contact with the exposed slide film's emulsion and "flashing" the image onto the dupe.

To learn more about "vintage" slide duplication techniques you could check out manuals for those devices. Beseler dual mode slide duplicator - A camera based duplicator Polaroid Polaprinter - Duplicated slides onto Polaroid

It is also possible that this is a form of in-camera masking. White-light masking is very rare and generally only used in scientific devices.

An even less likely explanation is that your slides are negative duplicates of a negative (two negatives makes a positive.) If that were true, though, you would have likely figured it out since the borders would have the characteristic beige base+fog of a normal color negative. This has, to my knowledge, never been common practice but it is possible using the technique mentioned above.