A segment of DNA (deoxyribonucleic acid) containing a gene encodes to radioactive labeled element, which is used in chromosomal hybridization.
In addition, segment of DNA contains other nucleotides apart from the gene-encoding sequence. Moreover, these sequences are responsible for the radioactivity seen in around 5 places.
Similarly, a protein encoding gene or an exon codes, for a specific protein. Hence, there may be slight similarities with other sequences. But the rest of the DNA fragment can have similarities.
Since, 8% of the genome contains repetitive sequences in the fragment of DNA may contain a few repetitive sequences. This would be the main reason for the radioactivity observed in around 5 places in the genome.