In an extremely simplified version, LGPL says you can only use the software if you guarantee A and B, while GPL says you can only use the software if you guarantee A, B, and C. Since {A,B} is a subset of {A,B,C}, licensing the LGPL software under something that requires A, B, and C guarantees A and B and so is fine by the LGPL. However, since the LGPL doesn't require you to guarantee C, then licensing software under the LGPL will not maintain all the requirements you must maintain to use GPL software.